Add realtime output modality and transcript events (#17701)

- Add outputModality to thread/realtime/start and wire text/audio output
selection through app-server, core, API, and TUI.\n- Rename the realtime
transcript delta notification and add a separate transcript done
notification that forwards final text from item done without correlating
it with deltas.

codex-rs/app-server/README.md

@@ -154,7 +154,7 @@ Example with notification opt-out:
 - `thread/inject_items` — append raw Responses API items to a loaded thread’s model-visible history without starting a user turn; returns `{}` on success.
 - `turn/steer` — add user input to an already in-flight regular turn without starting a new turn; returns the active `turnId` that accepted the input. Review and manual compaction turns reject `turn/steer`.
 - `turn/interrupt` — request cancellation of an in-flight turn by `(thread_id, turn_id)`; success is an empty `{}` response and the turn finishes with `status: "interrupted"`.
-- `thread/realtime/start` — start a thread-scoped realtime session (experimental); returns `{}` and streams `thread/realtime/*` notifications. Omit `transport` for the websocket transport, or pass `{ "type": "webrtc", "sdp": "..." }` to create a WebRTC session from a browser-generated SDP offer; the remote answer SDP is emitted as `thread/realtime/sdp`.
+- `thread/realtime/start` — start a thread-scoped realtime session (experimental); pass `outputModality: "text"` or `outputModality: "audio"` to choose model output, returns `{}` and streams `thread/realtime/*` notifications. Omit `transport` for the websocket transport, or pass `{ "type": "webrtc", "sdp": "..." }` to create a WebRTC session from a browser-generated SDP offer; the remote answer SDP is emitted as `thread/realtime/sdp`.
 - `thread/realtime/appendAudio` — append an input audio chunk to the active realtime session (experimental); returns `{}`.
 - `thread/realtime/appendText` — append text input to the active realtime session (experimental); returns `{}`.
 - `thread/realtime/stop` — stop the active realtime session for the thread (experimental); returns `{}`.
@@ -628,6 +628,7 @@ Then send `offer.sdp` to app-server. Core uses `experimental_realtime_ws_backend
 ```json
 { "method": "thread/realtime/start", "id": 40, "params": {
     "threadId": "thr_123",
+    "outputModality": "audio",
     "prompt": "You are on a call.",
     "sessionId": null,
     "transport": { "type": "webrtc", "sdp": "v=0\r\no=..." }
@@ -952,7 +953,8 @@ The thread realtime API emits thread-scoped notifications for session lifecycle
 
 - `thread/realtime/started` — `{ threadId, sessionId }` once realtime starts for the thread (experimental).
 - `thread/realtime/itemAdded` — `{ threadId, item }` for raw non-audio realtime items that do not have a dedicated typed app-server notification, including `handoff_request` (experimental). `item` is forwarded as raw JSON while the upstream websocket item schema remains unstable.
-- `thread/realtime/transcriptUpdated` — `{ threadId, role, text }` whenever realtime transcript text changes (experimental). This forwards the live transcript delta from that realtime event, not the full accumulated transcript.
+- `thread/realtime/transcript/delta` — `{ threadId, role, delta }` for live realtime transcript deltas (experimental).
+- `thread/realtime/transcript/done` — `{ threadId, role, text }` when realtime emits the final full text for a transcript part (experimental).
 - `thread/realtime/outputAudio/delta` — `{ threadId, audio }` for streamed output audio chunks (experimental). `audio` uses camelCase fields (`data`, `sampleRate`, `numChannels`, `samplesPerChannel`).
 - `thread/realtime/error` — `{ threadId, message }` when realtime encounters a transport or backend error (experimental).
 - `thread/realtime/closed` — `{ threadId, reason }` when the realtime transport closes (experimental).