Add app-server transport layer with websocket support (#10693)

- Adds --listen <URL> to codex app-server with two listen modes:
      - stdio:// (default, existing behavior)
      - ws://IP:PORT (new websocket transport)
  - Refactors message routing to be connection-aware:
- Tracks per-connection session state (initialize/experimental
capability)
      - Routes responses/errors to the originating connection
- Broadcasts server notifications/requests to initialized connections
- Updates initialization semantics to be per connection (not
process-global), and updates app-server docs accordingly.
- Adds websocket accept/read/write handling (JSON-RPC per text frame,
ping/pong handling, connection lifecycle events).

Testing

- Unit tests for transport URL parsing and targeted response/error
routing.
  - New websocket integration test validating:
      - per-connection initialization requirements
      - no cross-connection response leakage
      - same request IDs on different connections route independently.

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -4,6 +4,7 @@ mod app_list;
 mod collaboration_mode_list;
 mod compaction;
 mod config_rpc;
+mod connection_handling_websocket;
 mod dynamic_tools;
 mod experimental_api;
 mod experimental_feature_list;