feat!: Add method for fetching extended card

# Description

This change adds support for a new JSON-RPC method `agent/getAuthenticatedExtendedCard` for fetching authenticated extended card.

This is only applicable if `AgentCard.supportsAuthenticatedExtendedCard` bit is set to `true` that allows fetching authenticated context specific agent card.

In the previous versions, the extended card was served via `{AgentCard.url}/../agent/authenticatedExtendedCard` endpoint. This endpoint is being removed in favor of the JSON-RPC method. This endpoint will be deprecated in the SDKs and will be removed in a future release.

Release-As: 0.3.0

Author: holtskinner

src/a2a/types.py

@@ -172,6 +172,27 @@ class AgentSkill(A2ABaseModel):
     """
 
 
+class AuthenticatedExtendedCardNotConfiguredError(A2ABaseModel):
+    """
+    An A2A-specific error indicating that the agent does not have an
+    Authenticated Extended Card configured
+    """
+
+    code: Literal[-32007] = -32007
+    """
+    The error code for when an authenticated extended card is not configured.
+    """
+    data: Any | None = None
+    """
+    A primitive or structured value containing additional information about the error.
+    This may be omitted.
+    """
+    message: str | None = 'Authenticated Extended Card not configured'
+    """
+    The error message.
+    """
+
+
 class AuthorizationCodeOAuthFlow(A2ABaseModel):
     """
     Defines configuration details for the OAuth 2.0 Authorization Code flow.
@@ -375,6 +396,27 @@ class FileWithUri(A2ABaseModel):
     """
 
 
+class GetAuthenticatedExtendedCardRequest(A2ABaseModel):
+    """
+    Represents a JSON-RPC request for the `agent/getAuthenticatedExtendedCard` method.
+    """
+
+    id: str | int
+    """
+    The identifier for this request.
+    """
+    jsonrpc: Literal['2.0'] = '2.0'
+    """
+    The version of the JSON-RPC protocol. MUST be exactly "2.0".
+    """
+    method: Literal['agent/getAuthenticatedExtendedCard'] = (
+        'agent/getAuthenticatedExtendedCard'
+    )
+    """
+    The method name. Must be 'agent/getAuthenticatedExtendedCard'.
+    """
+
+
 class GetTaskPushNotificationConfigParams(A2ABaseModel):
     """
     Defines parameters for fetching a specific push notification configuration for a task.
@@ -999,6 +1041,7 @@ class A2AError(
         | UnsupportedOperationError
         | ContentTypeNotSupportedError
         | InvalidAgentResponseError
+        | AuthenticatedExtendedCardNotConfiguredError
     ]
 ):
     root: (
@@ -1013,6 +1056,7 @@ class A2AError(
         | UnsupportedOperationError
         | ContentTypeNotSupportedError
         | InvalidAgentResponseError
+        | AuthenticatedExtendedCardNotConfiguredError
     )
     """
     A discriminated union of all standard JSON-RPC and A2A-specific error types.
@@ -1170,6 +1214,7 @@ class JSONRPCErrorResponse(A2ABaseModel):
         | UnsupportedOperationError
         | ContentTypeNotSupportedError
         | InvalidAgentResponseError
+        | AuthenticatedExtendedCardNotConfiguredError
     )
     """
     An object describing the error that occurred.
@@ -1625,6 +1670,7 @@ class A2ARequest(
         | TaskResubscriptionRequest
         | ListTaskPushNotificationConfigRequest
         | DeleteTaskPushNotificationConfigRequest
+        | GetAuthenticatedExtendedCardRequest
     ]
 ):
     root: (
@@ -1637,6 +1683,7 @@ class A2ARequest(
         | TaskResubscriptionRequest
         | ListTaskPushNotificationConfigRequest
         | DeleteTaskPushNotificationConfigRequest
+        | GetAuthenticatedExtendedCardRequest
     )
     """
     A discriminated union representing all possible JSON-RPC 2.0 requests supported by the A2A specification.
@@ -1750,6 +1797,25 @@ class AgentCard(A2ABaseModel):
     """
 
 
+class GetAuthenticatedExtendedCardSuccessResponse(A2ABaseModel):
+    """
+    Represents a successful JSON-RPC response for the `agent/getAuthenticatedExtendedCard` method.
+    """
+
+    id: str | int | None = None
+    """
+    The identifier established by the client.
+    """
+    jsonrpc: Literal['2.0'] = '2.0'
+    """
+    The version of the JSON-RPC protocol. MUST be exactly "2.0".
+    """
+    result: AgentCard
+    """
+    The result is an Agent Card object.
+    """
+
+
 class Task(A2ABaseModel):
     """
     Represents a single, stateful operation or conversation between a client and an agent.
@@ -1769,7 +1835,7 @@ class Task(A2ABaseModel):
     """
     id: str
     """
-    A unique identifier for the task, generated by the client for a new task or provided by the agent.
+    A unique identifier for the task, generated by the server for a new task.
     """
     kind: Literal['task'] = 'task'
     """
@@ -1804,6 +1870,17 @@ class CancelTaskSuccessResponse(A2ABaseModel):
     """
 
 
+class GetAuthenticatedExtendedCardResponse(
+    RootModel[
+        JSONRPCErrorResponse | GetAuthenticatedExtendedCardSuccessResponse
+    ]
+):
+    root: JSONRPCErrorResponse | GetAuthenticatedExtendedCardSuccessResponse
+    """
+    Represents a JSON-RPC response for the `agent/getAuthenticatedExtendedCard` method.
+    """
+
+
 class GetTaskSuccessResponse(A2ABaseModel):
     """
     Represents a successful JSON-RPC response for the `tasks/get` method.
@@ -1889,6 +1966,7 @@ class JSONRPCResponse(
         | GetTaskPushNotificationConfigSuccessResponse
         | ListTaskPushNotificationConfigSuccessResponse
         | DeleteTaskPushNotificationConfigSuccessResponse
+        | GetAuthenticatedExtendedCardSuccessResponse
     ]
 ):
     root: (
@@ -1901,6 +1979,7 @@ class JSONRPCResponse(
         | GetTaskPushNotificationConfigSuccessResponse
         | ListTaskPushNotificationConfigSuccessResponse
         | DeleteTaskPushNotificationConfigSuccessResponse
+        | GetAuthenticatedExtendedCardSuccessResponse
     )
     """
     A discriminated union representing all possible JSON-RPC 2.0 responses