Feature request: Support OpenAI transcription-only realtime sessions

[maxwoedl]

I want to build a live transcription UI - mic in, streaming text out, no assistant, no turns.

OpenAI supports this natively with a realtime session of type: "transcription" and turn_detection: null, which streams conversation.item.input_audio_transcription.delta events continuously instead of emitting one transcript per detected turn. Models like gpt-live-transcribe are only reachable this way.

The raw session I currently mint by hand:

{
  "session": {
    "type": "transcription",
    "audio": {
      "input": {
        "format": { "type": "audio/pcm", "rate": 24000 },
        "transcription": { "model": "gpt-live-transcribe" },
        "turn_detection": null
      }
    }
  }
}

Why it isn't expressible today

The realtime layer is hardcoded to conversational voice chat at four points:

1. src/realtime/token.ts:22: buildClientSecretRequest() always sends { session: { type: 'realtime', model } }. No way to mint a transcription session.
2. src/realtime/types.ts:24: OpenAIRealtimeModel = 'gpt-realtime' | 'gpt-realtime-mini'. Transcription models aren't in the union (and wouldn't belong there anyway - in a transcription session the model goes under audio.input.transcription.model, not session.model).
3. src/realtime/session-update.ts:21: buildSessionUpdate() hardcodes transcription: { model: 'whisper-1' } and never forwards RealtimeSessionConfig.providerOptions, so there's no escape hatch to override it.
4. src/realtime/adapter.ts:247: only conversation.item.input_audio_transcription.completed is handled. There's no …transcription.delta case, so user speech only ever surfaces as isFinal: true, one event per committed turn.

vadMode: 'manual' does send turn_detection: null, but it's still turn-shaped, and still paired with an assistant that responds.

Proposed API

// server
const token = await realtimeToken({
  adapter: openaiRealtimeToken({
    sessionType: 'transcription',
    transcriptionModel: 'gpt-live-transcribe',
    vadMode: 'manual', // → turn_detection: null
  }),
})