## Why
Clients that create and negotiate realtime calls themselves need Codex to join
those calls without creating another call or overwriting the client-owned
session configuration.
## What changed
- Add the `existingCall` transport to `thread/realtime/start`, accepting a
client-provided `callId` and optional `realtimeSessionId`.
- Attach Codex over the call's sideband WebSocket without SDP negotiation or a
session update, and reconnect that sideband while preserving transcript
state.
- Default existing calls to realtime v1, support v1 and v3, and reject v2 or
options that would reconfigure the client-owned session.
- Encode call IDs as a single URL path segment for v3 sideband connections.
## Testing
Add protocol, app-server, core, reconnection, authentication-header, and URL
encoding coverage for existing-call attachments.
GitOrigin-RevId: 2046b70566f98efa55381bb1e461ea3cc68256c5
536f86e5cc