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/rollout/src/policy.rs

@@ -132,6 +132,7 @@ fn event_msg_persistence_mode(ev: &EventMsg) -> Option<EventPersistenceMode> {
         | EventMsg::DynamicToolCallResponse(_) => Some(EventPersistenceMode::Extended),
         EventMsg::Warning(_)
         | EventMsg::RealtimeConversationStarted(_)
+        | EventMsg::RealtimeConversationSdp(_)
         | EventMsg::RealtimeConversationRealtime(_)
         | EventMsg::RealtimeConversationClosed(_)
         | EventMsg::ModelReroute(_)