Review fixes for experimental streaming transcription

[gr2m]

Background

Follow-up to #16338 with fixes for the issues found during review. Targets the PR branch so the changes can be reviewed/merged into the feature PR before it lands on main.

Summary

Correctness

• experimental_streamTranscribe now pipes the model stream through a TransformStream instead of eagerly draining it inside ReadableStream.start(). This preserves consumer backpressure (previously every part of a long live session was buffered in memory regardless of consumer pace) and propagates fullStream cancellation to the model stream (previously a for await … break left the pump running, rejected all result promises with an internal TypeError, and never cancelled the provider stream).
• The OpenAI and xAI doStream streams define cancel() handlers: cancelling closes the WebSocket and stops reading the audio stream. Previously the connection and the audio-sending loop kept running (connection leak, continued audio upload/billing).
• xAI STT error events are now terminal and surface the actual server message. Previously the error was only enqueued as a stream part; when the server then closed the socket the caller got an unrelated AI_NoTranscriptGeneratedError, and when it didn't, result.text hung forever.
• OpenAI no longer enqueues an error part immediately before controller.error() — erroring a stream resets its queue, so the part was discarded unless a read was pending at that exact moment.
• Result promises that already resolved (e.g. warnings at stream-start) no longer flip to rejected when the stream fails later; only pending promises are rejected.
• The xAI WebSocket failure message now explains that native WebSocket implementations (browsers, Node.js, Deno, Bun) cannot send the Authorization header xAI requires, and points at createXai({ webSocket }). Without this, the default path fails with an opaque generic error on every standard runtime.
• Dated gpt-realtime-whisper-* snapshot IDs are accepted via prefix match instead of exact-match rejection.
• Warnings are emitted for non-streaming OpenAI options that were silently dropped (prompt, temperature, timestampGranularities, include) and for unrecognized inputAudioFormat.type values that xAI silently mapped to raw PCM (garbled transcripts with no diagnostic).
• Removed the duplicate transcript-final part xAI emitted for the same utterance on transcript.done (consumers rendered every utterance twice with no way to dedupe).

API surface

• The new provider spec stream types are exported as Experimental_TranscriptionModelV4Stream* at the package boundary, matching the Experimental_VideoModelV4/Experimental_RealtimeModelV4 pattern from contributing/project-philosophies.md for experimental spec surfaces. (Open question for maintainers: whether doStream? on the stable TranscriptionModelV4 needs further isolation — see review discussion.)
• streamTranscribe is declared unprefixed and aliased to experimental_streamTranscribe at the export seam per contributing/naming-conventions.md.
• StreamTranscriptionResult exposes language and durationInSeconds — both were delivered by the providers in the finish part but unreachable through the API (parity with transcribe()).
• The UnsupportedFunctionalityError for missing doStream explains the gateway/string-model limitation.
• readWebSocketMessageText and toWebSocketUrl moved to @ai-sdk/provider-utils (previously duplicated verbatim in both provider packages).
• The new user-facing xAI streaming options use .optional() instead of .nullish() per the provider-options guidance in the repo docs.

Tests, docs, examples

• New tests: cancellation propagation (core + both providers), server error events, unsupported-option warnings, dated snapshot IDs, promise settlement semantics, unrecognized audio format warning.
• New API reference page for experimental_streamTranscribe (+ index entry) — previously the function had no reference docs.
• Capability table corrections: gpt-realtime-whisper does not detect language (the code echoes the user's own language option); note that xAI streaming does not surface word timestamps/diarization/segments.
• Examples: examples/ai-functions/src/stream-transcribe/{openai,xai}/basic.ts (self-contained: synthesize PCM via generateSpeech, then stream-transcribe it). Adds ws to the example package for the xAI header requirement.

Not addressed (needs maintainer decision)

• Gateway support for streaming transcription (tracked as future work in Add experimental streaming transcription support #16338).
• Azure: gpt-realtime-whisper is in the shared model ID union but Azure has no webSocket setting and its api-key auth can never reach the WebSocket path.
• Stream part naming (transcript-partial/transcript-final vs the noun-verb convention) — worth settling before the strings freeze, but a rename touches the whole PR.
• Audio send loops have no bufferedAmount flow control; large pre-recorded sources buffer in the socket.

Verification

• pnpm --filter ai test:node (3174 passed)
• pnpm --filter @ai-sdk/openai test:node (741 passed)
• pnpm --filter @ai-sdk/xai test:node (368 passed)
• pnpm --filter @ai-sdk/provider-utils test:node (655 passed)
• pnpm --filter @ai-sdk/provider --filter @ai-sdk/provider-utils --filter @ai-sdk/openai --filter @ai-sdk/xai --filter ai type-check
• pnpm check