Add WebRTC transport to realtime start (#16960)

Adds WebRTC startup to the experimental app-server
`thread/realtime/start` method with an optional transport enum. The
websocket path remains the default; WebRTC offers create the realtime
session through the shared start flow and emit the answer SDP via
`thread/realtime/sdp`.

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

codex-rs/codex-api/src/endpoint/realtime_websocket/methods_v1.rs

@@ -40,7 +40,9 @@ pub(super) fn conversation_handoff_append_message(
 
 pub(super) fn session_update_session(instructions: String) -> SessionUpdateSession {
     SessionUpdateSession {
+        id: None,
         r#type: SessionType::Quicksilver,
+        model: None,
         instructions: Some(instructions),
         output_modalities: None,
         audio: SessionAudio {