## What changed
- Add an in-process `sideband_base_url` override to `ConversationStartTransport::ExistingCall`, while keeping app-server requests on the configured or default endpoint.
- Prefer the per-call endpoint when attaching and reconnecting a sideband, falling back to `experimental_realtime_ws_base_url` when no override is supplied.
- Preserve runtime authentication headers on the selected endpoint without adding bearer authorization.
## Testing
- Cover configured and per-call endpoint selection, authentication headers, call-specific handshake paths, and reconnect behavior.
GitOrigin-RevId: 16c439fe0a9ff8bcdda4ec615bb9983e30935052
## 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
Zahan Malkani