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/mod.rs

@@ -2,6 +2,7 @@ pub mod aggregate;
 pub mod compact;
 pub mod memories;
 pub mod models;
+pub mod realtime_websocket;
 pub mod responses;
 pub mod responses_websocket;
 mod session;