refactor(client): move agent card signature verification to BaseClient

[guglielmo-san]

This PR refactors the signature verification logic for Extended Agent Cards to be handled at the client level (BaseClient) rather than being duplicated across each individual transport implementation (GrpcTransport, JsonRpcTransport, RestTransport).

Changes:

• BaseClient: Added signature verification step and state mutation logic (self._card = card) inside get_extended_agent_card().
• Transports (gRPC, JSON-RPC, REST, TenantDecorator): Removed the signature_verifier parameter from get_extended_agent_card().
• Removed internal caching mechanism and state (self.agent_card, self._needs_extended_card) from the get_extended_agent_card() flow.

[gemini-code-assist]

Summary of Changes

Hello, 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 refactoring centralizes the signature verification process for extended agent cards within the BaseClient, streamlining the client architecture by removing redundant logic from individual transport layers. This change simplifies the get_extended_agent_card methods in transports and enhances maintainability by consolidating a critical security step.

Highlights

• Centralized Signature Verification: Moved the signature verification logic for Extended Agent Cards from individual transport implementations (gRPC, JSON-RPC, REST) to the BaseClient's get_extended_agent_card method, ensuring a single point of control for this critical step.
• Simplified Transport Interfaces: Removed the signature_verifier parameter from the get_extended_agent_card method across all transport implementations (GrpcTransport, JsonRpcTransport, RestTransport, TenantDecorator) and the Client class, simplifying their interfaces.
• Removed Redundant Caching: Eliminated internal caching mechanisms and state (self.agent_card, self._needs_extended_card) related to extended agent cards within the transport layers, as this state is now managed at the BaseClient level.
• Updated Integration Tests: Adjusted integration tests to align with the refactored signature verification and caching logic, removing outdated assertions and modifying how the signature_verifier is invoked.

Changelog

• src/a2a/client/base_client.py
  • Moved signature verification logic from transport layer to BaseClient's get_extended_agent_card method.
• src/a2a/client/client.py
  • Removed signature_verifier parameter from get_extended_agent_card.
• src/a2a/client/transports/base.py
  • Removed Callable import.
  • Removed signature_verifier parameter from get_extended_agent_card.
• src/a2a/client/transports/grpc.py
  • Removed _needs_extended_card initialization.
  • Removed signature_verifier parameter and associated verification/caching logic from get_extended_agent_card.
  • Added an early return for cards without extended capabilities.
• src/a2a/client/transports/jsonrpc.py
  • Removed Callable import.
  • Removed _needs_extended_card initialization.
  • Removed signature_verifier parameter and associated verification/caching logic from get_extended_agent_card.
• src/a2a/client/transports/rest.py
  • Removed Callable import.
  • Removed _needs_extended_card initialization.
  • Removed signature_verifier parameter and associated verification/caching logic from get_extended_agent_card.
• src/a2a/client/transports/tenant_decorator.py
  • Removed Callable import.
  • Removed signature_verifier parameter from get_extended_agent_card.
• src/a2a/compat/v0_3/grpc_transport.py
  • Removed signature_verifier parameter and associated verification logic from get_extended_agent_card.
• tests/integration/test_client_server_integration.py
  • Removed assertions related to transport.agent_card and _needs_extended_card.
  • Adjusted get_extended_agent_card calls to remove signature_verifier argument and explicitly call the verifier.

Activity

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[gemini-code-assist]

Code Review

This pull request refactors the agent card signature verification by moving the logic from individual transports to the BaseClient. The changes are mostly correct and align with the goal of centralizing this logic.

I've found a couple of issues:

1. A type signature mismatch in Client and BaseClient that would likely fail mypy checks.
2. A critical bug in GrpcTransport that could lead to a runtime AttributeError when handling an agent card.

I've provided specific comments and suggestions to address these points. Once fixed, the refactoring will be much more robust.