Add Frameless Bidi support for realtime conversations (#33261)

## What changed

- Add realtime conversation version `v3`, which preserves the V1 Codex Voice behavior while using Frameless Bidi `delegation.*` events.
- Translate audio, transcripts, handoffs, session context, and lifecycle events between the app server and the Frameless Bidi wire protocol.
- Support `v3` over WebSocket and WebRTC, including the Frameless `/live` endpoint, session configuration, headers, and default model selection.
- Update the app-server protocol schemas and documentation for the new version.

## Testing

- Add unit coverage for Frameless event parsing, outbound messages, context chunking, URL construction, and call creation.
- Add app-server end-to-end coverage for WebSocket delegation and WebRTC session startup.

GitOrigin-RevId: 79a3307bc209e1a54582ebd2febc07c909fca016

codex-rs/app-server-protocol/schema/json/v2/ThreadRealtimeStartedNotification.json

@@ -4,7 +4,8 @@
     "RealtimeConversationVersion": {
       "enum": [
         "v1",
-        "v2"
+        "v2",
+        "v3"
       ],
       "type": "string"
     }