feat!: Add method for fetching extended card

[a2a-bot]

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.

Protocol Update: a2aproject/A2A@2cd7d98

Release-As: 0.3.0

[gemini-code-assist]

Summary of Changes

Hello @a2a-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request updates the A2A specification by incorporating new types and methods related to an "Authenticated Extended Card" feature. It introduces a new error type, a new request type, and corresponding success and combined response types for retrieving this card. Additionally, it integrates these new types into the existing A2A JSON-RPC request and response structures and clarifies a description in the Task model.

Highlights

• New Error Type: A new error type, AuthenticatedExtendedCardNotConfiguredError, has been introduced to specifically handle scenarios where an authenticated extended card is not configured, providing a clear error code and message.
• New JSON-RPC Request: The specification now includes GetAuthenticatedExtendedCardRequest, a new JSON-RPC request type that defines the structure for clients to request an authenticated extended card from an agent.
• New JSON-RPC Responses: Corresponding success and combined response types, GetAuthenticatedExtendedCardSuccessResponse and GetAuthenticatedExtendedCardResponse, have been added to handle the outcomes of the new agent/getAuthenticatedExtendedCard method, ensuring proper data serialization for both successful retrievals and errors.
• Specification Integration: Core A2A union types such as A2AError, JSONRPCErrorResponse, A2ARequest, and JSONRPCResponse have been updated to seamlessly integrate the newly defined error, request, and response types, maintaining the integrity of the overall specification.
• Documentation Clarification: A minor but important clarification has been made to the description of the id field within the Task model, specifying that the task ID is now generated by the server.

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[gemini-code-assist]

Code Review

This pull request introduces the necessary Pydantic models for the new agent/getAuthenticatedExtendedCard JSON-RPC method, including request, response, and error types. The changes correctly integrate these new types into the existing type hierarchy.

However, the pull request appears to be incomplete. To make this feature functional and maintainable, the following components are missing:

• Tests: There are no new tests in tests/test_types.py to validate the new Pydantic models. Adding tests is crucial to prevent future regressions.
• Server-side Implementation: The new RPC method is not handled on the server. This would require changes in src/a2a/server/request_handlers/jsonrpc_handler.py and src/a2a/server/apps/jsonrpc/jsonrpc_app.py.
• Client-side Implementation: The client in src/a2a/client/client.py has not been updated to include a method for calling agent/getAuthenticatedExtendedCard.

While this might be the first in a series of PRs, it's generally better to include the implementation and tests for a new feature in a single, self-contained pull request. Please add the missing components or clarify the plan for introducing them.