Replace realtime websocket transport with WebRTC

Move the realtime model transport implementation to WebRTC while keeping the core session/event interface intact for the TUI layer.

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

codex-rs/codex-api/src/endpoint/realtime_websocket/methods_v2.rs

@@ -126,7 +126,3 @@ pub(super) fn session_update_session(
         },
     }
 }
-
-pub(super) fn websocket_intent() -> Option<&'static str> {
-    None
-}