realtime: rename provider session ids (#20361)

## Summary

Codex is repurposing `session` to mean a thread group, so the realtime
provider session id should no longer use `session_id` / `sessionId` in
Codex-facing protocol payloads. This PR renames that provider-specific
field to `realtime_session_id` / `realtimeSessionId` and intentionally
breaks clients that still send the old field names.

## What Changed

- Renamed realtime provider session fields in `ConversationStartParams`,
`RealtimeConversationStartedEvent`, and `RealtimeEvent::SessionUpdated`.
- Renamed app-server v2 realtime request and notification fields to
`realtimeSessionId`.
- Removed legacy serde aliases for `session_id` / `sessionId`; clients
must send the new names.
- Propagated the rename through core realtime startup, app-server
adapters, codex-api websocket handling, and TUI realtime state.
- Regenerated app-server protocol schema/TypeScript outputs and updated
app-server README examples.
- Kept upstream Realtime API concepts unchanged: provider `session.id`
parsing and `x-session-id` headers still use the upstream wire names.

## Testing

- CI is running on the latest pushed commit.
- Earlier local verification on this PR:
  - `cargo test -p codex-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-core
realtime_conversation`
  - `cargo test -p codex-app-server-protocol`
- `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-app-server
realtime_conversation`
- attempted `CODEX_SKIP_VENDORED_BWRAP=1 cargo test -p codex-tui` (local
linker bus error while linking the test binary)

---------

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

codex-rs/app-server/README.md

@@ -761,14 +761,14 @@ const offer = await pc.createOffer();
 await pc.setLocalDescription(offer);
 ```
 
-Then send `offer.sdp` to app-server. Core uses `experimental_realtime_ws_backend_prompt` for the backend instructions and the thread conversation id for the realtime session id. The start response is `{}`; the remote answer SDP arrives later as `thread/realtime/sdp` and should be passed to `setRemoteDescription()`:
+Then send `offer.sdp` to app-server. Core uses `experimental_realtime_ws_backend_prompt` for the backend instructions and the thread conversation id as the default Realtime API session identifier. This `realtimeSessionId` value refers to the upstream Realtime API session, not a Codex session/thread-group id. The start response is `{}`; the remote answer SDP arrives later as `thread/realtime/sdp` and should be passed to `setRemoteDescription()`:
 
 ```json
 { "method": "thread/realtime/start", "id": 40, "params": {
     "threadId": "thr_123",
     "outputModality": "audio",
     "prompt": "You are on a call.",
-    "sessionId": null,
+    "realtimeSessionId": null,
     "transport": { "type": "webrtc", "sdp": "v=0\r\no=..." }
 } }
 { "id": 40, "result": {} }
@@ -1100,7 +1100,7 @@ The fuzzy file search session API emits per-query notifications:
 
 The thread realtime API emits thread-scoped notifications for session lifecycle and streaming media:
 
-- `thread/realtime/started` — `{ threadId, sessionId }` once realtime starts for the thread (experimental).
+- `thread/realtime/started` — `{ threadId, realtimeSessionId }` once realtime starts for the thread (experimental). `realtimeSessionId` is the upstream Realtime API session identifier, not a Codex session/thread-group id.
 - `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/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).