## 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