codex-api: realtime websocket session.create + typed inbound events (#12036)

## Summary
- add realtime websocket client transport in codex-api
- send session.create on connect with backend prompt and optional
conversation_id
- keep session.update for prompt changes after connect
- switch inbound event parsing to a tagged enum (typed variants instead
of optional field bag)
- add a websocket e2e integration test in
codex-rs/codex-api/tests/realtime_websocket_e2e.rs

## Why
This moves the realtime transport to an explicit session-create
handshake and improves protocol safety with typed inbound events.

## Testing
- Added e2e integration test coverage for session create + event flow in
the API crate.

codex-rs/codex-api/src/endpoint/realtime_websocket/mod.rs

@@ -0,0 +1,10 @@
+pub mod methods;
+pub mod protocol;
+
+pub use methods::RealtimeWebsocketClient;
+pub use methods::RealtimeWebsocketConnection;
+pub use methods::RealtimeWebsocketEvents;
+pub use methods::RealtimeWebsocketWriter;
+pub use protocol::RealtimeAudioFrame;
+pub use protocol::RealtimeEvent;
+pub use protocol::RealtimeSessionConfig;