feat: seal api and clean up a2aclient tests

src/A2A/JsonRpc/ErrorTypes.cs

@@ -21,7 +21,7 @@ public A2AClientException(string message, Exception? innerException = null)
 /// <summary>
 /// Exception for HTTP errors.
 /// </summary>
-public class A2AClientHTTPException : A2AClientException
+public sealed class A2AClientHTTPException : A2AClientException
 {
     /// <summary>
     /// The HTTP status code.
@@ -49,7 +49,7 @@ public A2AClientHTTPException(HttpStatusCode statusCode, string message)
 /// <summary>
 /// Exception for JSON parsing errors.
 /// </summary>
-public class A2AClientJsonException : A2AClientException
+public sealed class A2AClientJsonException : A2AClientException
 {
     /// <summary>
     /// The error message.
@@ -70,7 +70,7 @@ public A2AClientJsonException(string message)
 /// <summary>
 /// Exception for missing API key.
 /// </summary>
-public class MissingAPIKeyException : Exception
+public sealed class MissingAPIKeyException : Exception
 {
     /// <summary>
     /// Creates a new missing API key error.

src/A2A/JsonRpc/JsonRpcRequest.cs

@@ -6,7 +6,7 @@ namespace A2A;
 /// <summary>
 /// Represents a JSON-RPC 2.0 Request object.
 /// </summary>
-public class JsonRpcRequest
+public sealed class JsonRpcRequest
 {
     /// <summary>
     /// Gets or sets the version of the JSON-RPC protocol.

src/A2A/JsonRpc/JsonRpcResponse.cs

@@ -8,7 +8,7 @@ namespace A2A;
 /// <summary>
 /// Represents a JSON-RPC 2.0 Response object.
 /// </summary>
-public class JsonRpcResponse
+public sealed class JsonRpcResponse
 {
     /// <summary>
     /// Gets or sets the version of the JSON-RPC protocol. MUST be exactly "2.0".

src/A2A/JsonRpc/MethodNotFoundError.cs

@@ -3,7 +3,7 @@ namespace A2A;
 /// <summary>
 /// Error for method not found.
 /// </summary>
-public class MethodNotFoundError : JsonRpcError
+public sealed class MethodNotFoundError : JsonRpcError
 {
     /// <summary>
     /// Creates a new method not found error.

src/A2A/Models/A2AResponse.cs

@@ -10,7 +10,7 @@ namespace A2A;
 [JsonDerivedType(typeof(TaskArtifactUpdateEvent), "artifact-update")]
 [JsonDerivedType(typeof(Message), "message")]
 [JsonDerivedType(typeof(AgentTask), "task")]
-public class A2AEvent
+public abstract class A2AEvent
 {
 }
 
@@ -20,6 +20,6 @@ public class A2AEvent
 [JsonPolymorphic(TypeDiscriminatorPropertyName = "kind")]
 [JsonDerivedType(typeof(Message), "message")]
 [JsonDerivedType(typeof(AgentTask), "task")]
-public class A2AResponse : A2AEvent
+public abstract class A2AResponse : A2AEvent
 {
 }

src/A2A/Models/AgentCapabilities.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Defines optional capabilities supported by an agent.
 /// </summary>
-public class AgentCapabilities
+public sealed class AgentCapabilities
 {
     /// <summary>
     /// Gets or sets a value indicating whether the agent supports SSE.

src/A2A/Models/AgentCard.cs

@@ -11,7 +11,7 @@ namespace A2A;
 /// - Default modalities/content types supported by the agent.
 /// - Authentication requirements.
 /// </remarks>
-public class AgentCard
+public sealed class AgentCard
 {
     /// <summary>
     /// Gets or sets the human readable name of the agent.

src/A2A/Models/AgentInterface.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Provides a declaration of a combination of target URL and supported transport to interact with an agent.
 /// </summary>
-public class AgentInterface
+public sealed class AgentInterface
 {
     /// <summary>
     /// The transport supported by this URL.

src/A2A/Models/AgentProvider.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Represents the service provider of an agent.
 /// </summary>
-public class AgentProvider
+public sealed class AgentProvider
 {
     /// <summary>
     /// Agent provider's organization name.

src/A2A/Models/AgentSkill.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Represents a unit of capability that an agent can perform.
 /// </summary>
-public class AgentSkill
+public sealed class AgentSkill
 {
     /// <summary>
     /// Unique identifier for the agent's skill.

src/A2A/Models/AgentTask.cs

@@ -6,7 +6,7 @@ namespace A2A;
 /// <summary>
 /// Represents a task that can be processed by an agent.
 /// </summary>
-public class AgentTask : A2AResponse
+public sealed class AgentTask : A2AResponse
 {
     /// <summary>
     /// Unique identifier for the task.

src/A2A/Models/AgentTaskStatus.cs

@@ -8,7 +8,7 @@ namespace A2A;
 /// <remarks>
 /// Contains the TaskState and accompanying message.
 /// </remarks>
-public class AgentTaskStatus
+public sealed class AgentTaskStatus
 {
     /// <summary>
     /// The current state of the task.

src/A2A/Models/Artifact.cs

@@ -6,7 +6,7 @@ namespace A2A;
 /// <summary>
 /// Represents an artifact generated for a task.
 /// </summary>
-public class Artifact
+public sealed class Artifact
 {
     /// <summary>
     /// Unique identifier for the artifact.

src/A2A/Models/AuthenticationInfo.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Defines authentication details for push notifications.
 /// </summary>
-public class AuthenticationInfo
+public sealed class AuthenticationInfo
 {
     /// <summary>
     /// Supported authentication schemes - e.g. Basic, Bearer.

src/A2A/Models/DataPart.cs

@@ -6,7 +6,7 @@ namespace A2A;
 /// <summary>
 /// Represents a structured data segment within a message part.
 /// </summary>
-public class DataPart : Part
+public sealed class DataPart : Part
 {
     /// <summary>
     /// Structured data content.

src/A2A/Models/FileContent.cs

@@ -20,7 +20,7 @@ public class FileContent
 /// <summary>
 /// Define the variant where 'bytes' is present and 'uri' is absent.
 /// </summary>
-public class FileWithBytes : FileContent
+public sealed class FileWithBytes : FileContent
 {
     /// <summary>
     /// Optional name for the file.
@@ -51,7 +51,7 @@ public class FileWithBytes : FileContent
 /// <summary>
 /// Define the variant where 'uri' is present and 'bytes' is absent.
 /// </summary>
-public class FileWithUri : FileContent
+public sealed class FileWithUri : FileContent
 {
     /// <summary>
     /// Optional name for the file.

src/A2A/Models/FilePart.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Represents a File segment within parts.
 /// </summary>
-public class FilePart : Part
+public sealed class FilePart : Part
 {
     /// <summary>
     /// File content either as url or bytes.

src/A2A/Models/Message.cs

@@ -22,7 +22,7 @@ public enum MessageRole
 /// <summary>
 /// JSON converter for MessageRole enum.
 /// </summary>
-public class MessageRoleConverter : JsonConverter<MessageRole>
+public sealed class MessageRoleConverter : JsonConverter<MessageRole>
 {
     /// <inheritdoc />
     public override MessageRole Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
@@ -52,7 +52,7 @@ public override void Write(Utf8JsonWriter writer, MessageRole value, JsonSeriali
 /// <summary>
 /// Represents a single message exchanged between user and agent.
 /// </summary>
-public class Message : A2AResponse
+public sealed class Message : A2AResponse
 {
     /// <summary>
     /// Message sender's role.

src/A2A/Models/MessageSendParams.cs

@@ -9,7 +9,7 @@ namespace A2A;
 /// <remarks>
 /// Sent by the client to the agent as a request. May create, continue or restart a task.
 /// </remarks>
-public class MessageSendParams
+public sealed class MessageSendParams
 {
     /// <summary>
     /// The message being sent to the server.
@@ -34,7 +34,7 @@ public class MessageSendParams
 /// <summary>
 /// Configuration for the send message request.
 /// </summary>
-public class MessageSendConfiguration
+public sealed class MessageSendConfiguration
 {
     /// <summary>
     /// Accepted output modalities by the client.

src/A2A/Models/PushNotificationConfig.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Configuration for setting up push notifications for task updates.
 /// </summary>
-public class PushNotificationConfig
+public sealed class PushNotificationConfig
 {
     /// <summary>
     /// URL for sending the push notifications.

src/A2A/Models/SecurityScheme.cs

@@ -19,7 +19,7 @@ public class SecurityScheme
 /// <summary>
 /// API Key security scheme.
 /// </summary>
-public class ApiKeySecurityScheme : SecurityScheme
+public sealed class ApiKeySecurityScheme : SecurityScheme
 {
     /// <summary>
     /// The name of the header, query or cookie parameter to be used.
@@ -39,7 +39,7 @@ public class ApiKeySecurityScheme : SecurityScheme
 /// <summary>
 /// HTTP Authentication security scheme.
 /// </summary>
-public class HttpAuthSecurityScheme : SecurityScheme
+public sealed class HttpAuthSecurityScheme : SecurityScheme
 {
     /// <summary>
     /// The name of the HTTP Authentication scheme to be used in the Authorization header as defined in RFC7235.
@@ -66,7 +66,7 @@ public class HttpAuthSecurityScheme : SecurityScheme
 /// <summary>
 /// OAuth2.0 security scheme configuration.
 /// </summary>
-public class OAuth2SecurityScheme : SecurityScheme
+public sealed class OAuth2SecurityScheme : SecurityScheme
 {
     /// <summary>
     /// An object containing configuration information for the flow types supported.
@@ -79,7 +79,7 @@ public class OAuth2SecurityScheme : SecurityScheme
 /// <summary>
 /// OpenID Connect security scheme configuration.
 /// </summary>
-public class OpenIdConnectSecurityScheme : SecurityScheme
+public sealed class OpenIdConnectSecurityScheme : SecurityScheme
 {
     /// <summary>
     /// Well-known URL to discover the [[OpenID-Connect-Discovery]] provider metadata.
@@ -92,7 +92,7 @@ public class OpenIdConnectSecurityScheme : SecurityScheme
 /// <summary>
 /// Allows configuration of the supported OAuth Flows.
 /// </summary>
-public class OAuthFlows
+public sealed class OAuthFlows
 {
     /// <summary>
     /// Configuration for the OAuth Authorization Code flow.
@@ -128,7 +128,7 @@ public class OAuthFlows
 /// <summary>
 /// Configuration details for a supported OAuth Flow.
 /// </summary>
-public class AuthorizationCodeOAuthFlow
+public sealed class AuthorizationCodeOAuthFlow
 {
     /// <summary>
     /// The authorization URL to be used for this flow.
@@ -173,7 +173,7 @@ public class AuthorizationCodeOAuthFlow
 /// <summary>
 /// Configuration details for a supported OAuth Flow.
 /// </summary>
-public class ClientCredentialsOAuthFlow
+public sealed class ClientCredentialsOAuthFlow
 {
     /// <summary>
     /// The token URL to be used for this flow.
@@ -204,7 +204,7 @@ public class ClientCredentialsOAuthFlow
 /// <summary>
 /// Configuration details for a supported OAuth Flow.
 /// </summary>
-public class PasswordOAuthFlow
+public sealed class PasswordOAuthFlow
 {
     /// <summary>
     /// The token URL to be used for this flow.
@@ -239,7 +239,7 @@ public class PasswordOAuthFlow
 /// <summary>
 /// Configuration details for a supported OAuth Flow.
 /// </summary>
-public class ImplicitOAuthFlow
+public sealed class ImplicitOAuthFlow
 {
     /// <summary>
     /// The authorization URL to be used for this flow.

src/A2A/Models/TaskArtifactUpdateEvent.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Sent by server during sendStream or subscribe requests.
 /// </summary>
-public class TaskArtifactUpdateEvent : TaskUpdateEvent
+public sealed class TaskArtifactUpdateEvent : TaskUpdateEvent
 {
     /// <summary>
     /// Generated artifact.

src/A2A/Models/TaskPushNotificationConfig.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Parameters for setting or getting push notification configuration for a task.
 /// </summary>
-public class TaskPushNotificationConfig
+public sealed class TaskPushNotificationConfig
 {
     /// <summary>
     /// Task id.

src/A2A/Models/TaskQueryParams.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Parameters for querying a task, including optional history length.
 /// </summary>
-public class TaskQueryParams : TaskIdParams
+public sealed class TaskQueryParams : TaskIdParams
 {
     /// <summary>
     /// Number of recent messages to be retrieved.

src/A2A/Models/TaskStatusUpdateEvent.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Event sent by server during sendStream or subscribe requests.
 /// </summary>
-public class TaskStatusUpdateEvent : TaskUpdateEvent
+public sealed class TaskStatusUpdateEvent : TaskUpdateEvent
 {
     /// <summary>
     /// Gets or sets the current status of the task.

src/A2A/Models/TextPart.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Represents a text segment within parts.
 /// </summary>
-public class TextPart : Part
+public sealed class TextPart : Part
 {
     /// <summary>
     /// Gets or sets the text content.

src/A2A/Server/InMemoryTaskStore.cs

@@ -3,7 +3,7 @@ namespace A2A;
 /// <summary>
 /// In-memory implementation of task store for development and testing.
 /// </summary>
-public class InMemoryTaskStore : ITaskStore
+public sealed class InMemoryTaskStore : ITaskStore
 {
     private readonly Dictionary<string, AgentTask> _taskCache = [];
     private readonly Dictionary<string, TaskPushNotificationConfig> _pushNotificationCache = [];

src/A2A/Server/TaskManager.cs

@@ -9,7 +9,7 @@ namespace A2A;
 /// Helps manage a task's lifecycle during execution of a request, responsible for retrieving,
 /// saving, and updating the Task object based on events received from the agent.
 /// </remarks>
-public class TaskManager : ITaskManager
+public sealed class TaskManager : ITaskManager
 {
     /// <summary>
     /// OpenTelemetry ActivitySource for tracing.

src/A2A/Server/TaskUpdateEventEnumerator.cs

@@ -5,7 +5,7 @@ namespace A2A;
 /// <summary>
 /// Enumerator for streaming task update events to clients.
 /// </summary>
-public class TaskUpdateEventEnumerator : IAsyncEnumerable<A2AEvent>
+public sealed class TaskUpdateEventEnumerator : IAsyncEnumerable<A2AEvent>
 {
     private bool isFinal;
     private readonly ConcurrentQueue<A2AEvent> _UpdateEvents = new();