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/mcp-server/src/codex_tool_runner.rs

@@ -387,6 +387,7 @@ async fn run_codex_tool_session_inner(
                     | EventMsg::CollabResumeBegin(_)
                     | EventMsg::CollabResumeEnd(_)
                     | EventMsg::RealtimeConversationStarted(_)
+                    | EventMsg::RealtimeConversationSdp(_)
                     | EventMsg::RealtimeConversationRealtime(_)
                     | EventMsg::RealtimeConversationClosed(_)
                     | EventMsg::DeprecationNotice(_) => {