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`.

---------

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

codex-rs/core/tests/suite/compact_remote.rs

@@ -118,6 +118,7 @@ async fn start_realtime_conversation(codex: &codex_core::CodexThread) -> Result<
         .submit(Op::RealtimeConversationStart(ConversationStartParams {
             prompt: "backend prompt".to_string(),
             session_id: None,
+            transport: None,
         }))
         .await?;