Add codex tool support for realtime v2 handoff (#14554)

- Advertise a `codex` function tool in realtime v2 session updates.
- Emit handoff replies as `function_call_output` items while keeping v1
behavior unchanged.
- Split realtime event parsing into explicit v1/v2 modules with shared
common helpers.

---------

Co-authored-by: Codex <noreply@openai.com>

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

@@ -1,5 +1,7 @@
 pub mod methods;
 pub mod protocol;
+mod protocol_common;
+mod protocol_v1;
 mod protocol_v2;
 
 pub use codex_protocol::protocol::RealtimeAudioFrame;