[BUG] OpenAI STT broken — Realtime Beta API was sunset

Describe the bug OpenAI killed the Realtime Beta on May 12, 2026. Every STT session now closes immediately:

code: 'beta_api_shape_disabled',
message: 'The Realtime Beta API is no longer supported. Please use /v1/realtime for the GA API.'
WebSocket closed: 4000

Provider still sends the beta handshake (OpenAI-Beta: realtime=v1 + transcription_session.update). Nothing transcribes.

To Reproduce

1. Use OpenAI as STT.
2. Start a listen session.
3. Both sessions die instantly.

Expected behavior Transcription works.

Environment (please complete the following information):

• OS: Windows 11 / macOS (server-side error, platform-agnostic)
• App Version: `main` @ c55a489

Additional context GA migration: drop the beta header, switch to `session.update` with `session.type: "transcription"`, move audio config under `session.audio.input.*`, `format` becomes `{ type: "audio/pcm", rate: 24000 }`. Inbound event names unchanged. PR incoming.