Experimental realtime model support via gateway.experimental_realtime()

Patch Changes

• a3bb04a: feat(gateway): add experimental realtime model support

  Adds gateway.experimental_realtime() for bidirectional audio/text realtime sessions routed through the AI Gateway. Like every other Gateway modality, the client speaks the normalized AI SDK realtime protocol and the Gateway translates to/from the upstream provider server-side, so GatewayRealtimeModel is a thin identity codec. Gateway realtime is server-side only for v0 and throws if used in a browser because it returns the resolved Gateway auth token rather than a minted ephemeral client secret. Because the browser WebSocket API cannot set request headers, the Gateway auth token is carried via the Sec-WebSocket-Protocol subprotocol (the same workaround used for OpenAI) and the model id rides the ?ai-model-id= query — the WS transport of the ai-model-id header used by the HTTP routes. The model id is passed through verbatim; the Gateway owns resolution. Provider options (including BYOK) flow through the normalized session.update, exactly as they ride the request body on the non-realtime routes.

  The versioned subprotocol auth contract is centralized so the client and the Gateway server share one definition: getGatewayRealtimeProtocols (client encode) and getGatewayRealtimeAuthToken (server decode), plus the GATEWAY_REALTIME_SUBPROTOCOL / GATEWAY_AUTH_SUBPROTOCOL_PREFIX constants.

  GatewayProviderOptions documents the stable client-facing option fields while remaining open to service-owned options. Runtime validation lives in the Gateway service so the server can evolve without requiring an SDK release for every new option.