feat(websocket): add websocket cpb for a2a-rs

[hackeramitkumar]

Summary

This PR adds the initial WebSocket transport binding for a2a-rs, improves coverage around the new client/server behavior, and aligns push-notification handling with the current v1.0.0 A2A schema.

What Changed

• Added a2a-websocket support for the WEBSOCKET transport protocol.
• Implemented a WebSocket client transport that multiplexes:
  • unary requests
  • streaming requests
  • stream cancellation
  • connection teardown
• Implemented a WebSocket server router for Axum using the a2a.v1 subprotocol.
• Added WebSocket envelope handling for:
  • request/response messages
  • streaming events
  • streamEnd
  • structured WebSocket error envelopes
  • fatal protocol closes
• Added end-to-end tests that boot a real Axum server and connect with WebSocketTransport.
• Added focused unit tests for client, server, common protocol helpers, and error mapping.
• Switched the WebSocket client pending-state mutex to parking_lot::Mutex.
• Updated push-notification config usage to the current flat TaskPushNotificationConfig model.
• Added TRANSPORT_PROTOCOL_WEBSOCKET.

Notes

• wss:// endpoints are currently rejected with a clear unsupported error; TLS can be handled upstream for now.
• Binary WebSocket frames are rejected/reserved for future use.
• The client enforces a2a.v1 subprotocol negotiation.
• Stream drops send a cancelStream envelope to the server.

Verification

Validated locally with:

cargo fmt --all -- --check
cargo test -p a2a-lf --lib
cargo test -p a2a-websocket
cargo build --workspace

Results:

• a2a-lf lib tests: 141 passed
• a2a-websocket unit tests: 105 passed
• a2a-websocket e2e tests: 8 passed
• workspace build: passed

[gemini-code-assist]

Code Review

This pull request introduces the a2a-websocket crate, which implements the A2A v1 WebSocket protocol binding for both clients and servers. Key additions include a WebSocketTransport for multiplexed client requests and an axum::Router builder for serving A2A operations over persistent WebSocket connections. The reviewer identified several areas for improvement, including adding a timeout to connection attempts, implementing backpressure by replacing unbounded channels with bounded ones, and removing redundant streamEnd frames in the stream cancellation logic.

[codecov]

Codecov Report

❌ Patch coverage is 96.66174% with 113 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
a2a-websocket/src/server.rs	94.23%	71 Missing ⚠️
a2a-websocket/src/client.rs	96.64%	40 Missing ⚠️
a2a/src/types.rs	99.67%	2 Missing ⚠️

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