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/tui/src/chatwidget/realtime.rs

@@ -229,6 +229,7 @@ impl ChatWidget {
             ConversationStartParams {
                 prompt: REALTIME_CONVERSATION_PROMPT.to_string(),
                 session_id: None,
+                transport: None,
             },
         ));
         self.request_redraw();