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/core/src/codex.rs

@@ -7037,6 +7037,7 @@ fn realtime_text_for_event(msg: &EventMsg) -> Option<String> {
         EventMsg::Error(_)
         | EventMsg::Warning(_)
         | EventMsg::RealtimeConversationStarted(_)
+        | EventMsg::RealtimeConversationSdp(_)
         | EventMsg::RealtimeConversationRealtime(_)
         | EventMsg::RealtimeConversationClosed(_)
         | EventMsg::ModelReroute(_)