feat: seal api and clean up a2aclient tests (#37)

* make required props not optional, use jsonrequired attribute instead of required one, etc

* format test class

* change type of FilePart.File property from the specific FileWithBytes to the FileContent base one.

* seal classes

Author: SergeyMenshykh

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.