feat: Implement extended card support on server side

[guglielmo-san]

Description

This PR introduces a more robust implementation of the getAuthenticatedExtendedAgentCard API.
The user may inject in the default_request_handler an AgentCart object or a callback (ExtendedAgentCardProvider) which receives as input parameters the ServerCallContext and returns an AgentCard object.

• If extendedAgentCardProvider is provided as callback: The logic delegates entirely to the callback. This allows consumers to implement custom authorization check if the user has insufficient permissions.
• If extendedAgentCardProvider is provided as AgentCard object:
  1. User in context is Authenticated: Returns extendedAgentCard.
  2. User in context is Unauthenticated: Returns the basic agentCard.

Fixes #196 🦕

Release-As: 0.3.6

[gemini-code-assist]

Summary of Changes

Hello @guglielmo-san, 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 significantly enhances the server-side support for extended agent cards by introducing a flexible mechanism to modify these cards based on user authentication and other contextual information. It allows developers to inject custom logic via a new ExtendedCardModifier callback, ensuring that sensitive details in the extended agent card can be dynamically cleared or adjusted before being returned to the client, thereby improving security and customization capabilities.

Highlights

• New ExtendedCardModifier Type: A new type ExtendedCardModifier has been introduced, allowing custom logic to modify the extendedAgentCard based on the ServerCallContext.
• getAuthenticatedExtendedAgentCard API Enhancement: The getAuthenticatedExtendedAgentCard method in A2ARequestHandler and DefaultRequestHandler now accepts an optional ServerCallContext parameter, enabling context-aware card modification.
• Configurable Extended Card Modification: The DefaultRequestHandler constructor now accepts an optional extendedCardModifier callback, enabling server-side customization of the extended agent card based on user authentication levels or other contextual data.
• Robust Error Handling: Added checks to getAuthenticatedExtendedAgentCard to ensure the agent card explicitly supports extended cards and that an extended card is configured, throwing specific errors otherwise.
• JSON-RPC Transport Integration: The JSON-RPC transport handler has been updated to correctly route and handle calls to the enhanced agent/getAuthenticatedExtendedCard method, passing the ServerCallContext.
• Comprehensive Test Coverage: New unit tests have been added to validate the behavior of getAuthenticatedExtendedAgentCard under various scenarios, including unsupported operations, missing configuration, and successful authenticated retrieval with a custom modifier.

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[gemini-code-assist]

Code Review

This pull request introduces support for an authenticated extended agent card, which can reveal more details to authenticated users. This is primarily achieved by adding an extendedCardModifier callback to the DefaultRequestHandler. The changes look good overall, but I've identified a few critical issues. The ServerCallContext, which contains authentication information, is not being passed to getAuthenticatedExtendedAgentCard in the JSON-RPC transport handler. Additionally, the new tests for this feature contain a bug that makes them pass incorrectly and they are not type-safe. I've also suggested a design improvement to the ExtendedCardModifier signature to make it more robust and useful by providing it with access to the public agent card.