* Studio Hub: default downloads to Xet transport
Model and dataset downloads defaulted to HTTP; flip the default to Xet for faster parallel chunked transfers.
- Frontend: DEFAULT_TRANSPORT_MODE is now Xet, so a user with no saved preference starts on Xet. effectiveTransportMode() already downgrades to HTTP and warns when hf_xet is unavailable, so this degrades gracefully.
- Backend: DownloadModelRequest.use_xet and DownloadDatasetRequest.use_xet default to True, keeping the API in step with the UI. Set use_xet=False for sequential HTTP Range-resume.
- Align the internal _spawn_download_worker default so no caller silently falls back to HTTP.
Inference and training model loads were already Xet-first with an HTTP stall fallback, so this brings explicit downloads in line with the rest of Studio.
* Studio Hub: gracefully fall back to HTTP when Xet is unavailable
With Xet now the default, an omitted or explicit use_xet=True from a non-UI API caller would 400 on installs without hf_xet, since resolve_transport raises when the transport is unavailable.
Add resolve_effective_use_xet(), which downgrades a Xet request to HTTP (with a warning) when hf_xet is missing, mirroring the frontend's own downgrade. Both the model and dataset flows now derive a single effective use_xet and feed it to resolve_transport and spawn_worker, so the recorded transport and the worker env can never disagree. The UI is unaffected: it already resolves availability and passes use_xet explicitly.
* Add tests for resolve_effective_use_xet Xet to HTTP fallback
* Trim comments for PR #6433
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Co-authored-by: Daniel Han <danielhanchen@gmail.com>
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