Open a realtime session

Upgrades to a websocket and relays an OpenAI-compatible realtime (speech-to-speech) session to the provider that owns the model named in the ?model= query parameter. Provider credentials are injected by the gateway. Passing ?call_id= instead attaches to an existing WebRTC/SIP call as a sideband channel; calls created through this gateway instance are routed automatically, others need explicit model (and provider) parameters.