Open a realtime translation session

Upgrades to a websocket and relays an OpenAI-compatible realtime speech translation session (e.g. gpt-realtime-translate) to the provider that owns the model named in the ?model= query parameter. Translation sessions use their own event namespace and set the target language with session.update (session.audio.output.language); provider credentials are injected by the gateway. Equivalent to /v1/realtime?intent=translation.