refactor(client): allow transport agnostic per invocation timeouts

[ishymko]

Replace magic http_kwargs key with explicit timeout property on ClientCallContext.

Fixes #763.

[gemini-code-assist]

Summary of Changes

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This pull request refactors the client-side timeout mechanism to provide a more explicit and transport-agnostic way to specify per-invocation timeouts. By introducing a dedicated timeout property on the ClientCallContext, it eliminates the reliance on magic http_kwargs and centralizes timeout handling across gRPC, JSON-RPC, and REST transports, improving clarity and maintainability.

Highlights

• Per-invocation Timeout: Introduced a dedicated timeout property to ClientCallContext for specifying per-invocation timeouts, replacing the previous reliance on magic http_kwargs.
• gRPC Transport Refactor: Refactored the gRPC client transport to centralize call logic into new _call_grpc and _call_grpc_stream helper methods, which now consistently apply the ClientCallContext timeout.
• JSON-RPC and REST Transport Updates: Updated JSON-RPC and REST client transports to utilize the new timeout property from ClientCallContext for HTTP requests.
• Comprehensive Testing: Added new unit tests and updated existing ones across gRPC, JSON-RPC, and REST transports to ensure the correct propagation and application of the new per-invocation timeout.

Changelog

• src/a2a/client/middleware.py
  • Added a timeout: float | None field to the ClientCallContext class.
• src/a2a/client/transports/grpc.py
  • Replaced direct stub calls with new internal _call_grpc and _call_grpc_stream helper methods for all gRPC operations.
  • Implemented _get_grpc_timeout to retrieve the timeout from ClientCallContext.
  • Implemented _call_grpc to handle non-streaming gRPC calls, applying metadata and the new timeout.
  • Implemented _call_grpc_stream to handle streaming gRPC calls, applying metadata and the new timeout, and managing stream reading.
• src/a2a/client/transports/jsonrpc.py
  • Modified _get_http_args to construct http_kwargs with an httpx.Timeout object based on the ClientCallContext.timeout property, if present.
• src/a2a/client/transports/rest.py
  • Modified _get_http_args to construct http_kwargs with an httpx.Timeout object based on the ClientCallContext.timeout property, if present.
• tests/client/transports/test_grpc_client.py
  • Added test_send_message_with_timeout_context to verify that the ClientCallContext timeout is correctly passed to the gRPC stub.
  • Updated several existing test cases to explicitly include timeout=None in the ClientCallContext for consistency.
• tests/client/transports/test_jsonrpc_client.py
  • Added test_send_message_with_timeout_context to confirm that the ClientCallContext timeout is correctly applied to httpx.build_request.
• tests/client/transports/test_rest_client.py
  • Added test_send_message_with_timeout_context to confirm that the ClientCallContext timeout is correctly applied to httpx.build_request.

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[gemini-code-assist]

Code Review

This pull request refactors how client call timeouts are handled by introducing an explicit timeout property on ClientCallContext, replacing the use of a magic http_kwargs key. The changes are well-implemented across the gRPC, JSON-RPC, and REST transports, including good refactoring in the gRPC transport to reduce code duplication. The new functionality is also well-tested. I have one minor suggestion for the REST transport to ensure type consistency.