feat!: Add method for fetching extended card

[holtskinner]

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

[gemini-code-assist]

Summary of Changes

Hello @holtskinner, 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 primarily focuses on migrating the mechanism for fetching authenticated extended card information from an existing HTTP endpoint to a new, dedicated JSON-RPC method. This change aims to standardize the API's interaction patterns, improve clarity in data exchange, and enhance error handling by introducing specific data models and error types for the new JSON-RPC flow.

Highlights

• New JSON-RPC Method: I've introduced a new JSON-RPC method, agent/getAuthenticatedExtendedCard, to facilitate fetching authenticated extended card data. This new method is designed to replace an existing HTTP endpoint, streamlining the API's approach to this functionality.
• New Data Models: I've added several new Pydantic models, including GetAuthenticatedExtendedCardRequest, GetAuthenticatedExtendedCardSuccessResponse, and GetAuthenticatedExtendedCardResponse, to precisely define the structure for the new JSON-RPC request and its corresponding successful and overall responses.
• Enhanced Error Handling: I've implemented a specific error type, AuthenticatedExtendedCardNotConfiguredError, to provide clear and standardized feedback when an authenticated extended card is not configured for an agent, improving the API's error reporting.
• API Schema Integration: I've updated the core A2A JSON-RPC unions (A2ARequest, JSONRPCResponse, A2AError, JSONRPCErrorResponse) to seamlessly incorporate the newly defined request, response, and error types, ensuring the API schema remains consistent and up-to-date.

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[gemini-code-assist]

Code Review

This pull request introduces the necessary Pydantic types for a new agent/getAuthenticatedExtendedCard JSON-RPC method. The changes are well-structured and align with the existing patterns in the codebase. I've made one suggestion to improve documentation consistency.