- Replace unbounded `arrayBuffer()`/`blob()` calls in `download()` and
`downloadBlob()` with streaming reads that enforce a **2 GiB default
size limit**
- Add `abortSignal` passthrough from callers (`transcribe`,
`generateVideo`) to `fetch()`
- Check `Content-Length` header for early rejection before reading body
- Track bytes incrementally via `ReadableStream.getReader()`, abort with
`DownloadError` when limit exceeded
- Expose configurable `download` parameter on `transcribe()` and
`experimental_generateVideo()` (instead of adding a new
`maxDownloadSize` argument) — keeps download config separate from API
function signatures
- Export `createDownload({ maxBytes })` factory from `ai` for custom
size limits

closes #9481 / addresses
#9481 (comment)

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>