Pin Chatterbox model revisions and publish atomically

[Blackspirits]

Summary

Chatterbox model downloads currently use Hugging Face resolve/main URLs even though the code comments describe the Base V3 models as pinned. The GGUFs are also downloaded directly to their final paths, so a failed/partial download can leave a file that later existence checks treat as installed.

This change:

• pins Base Chatterbox model URLs to immutable Hugging Face revision c45504bb8d55473a2213db17ec472ed11b69056a
• pins Chatterbox Turbo model URLs to immutable Hugging Face revision b544cf8b49504d880640864a757b3ff3e4421a42
• removes resolve/main from Chatterbox model downloads
• downloads each GGUF to a same-directory .part path
• publishes the final path only after the download completes successfully and cancellation is rechecked
• removes stale/failed temporary files
• preserves an existing final model if a replacement attempt fails
• leaves model keys, file names, backend selection, quantizations and model contents unchanged
• adds regressions for both immutable revision URLs, successful atomic publication, and failed-download preservation

Validation

• current cstr/chatterbox-GGUF tree identifies revision c45504bb8d55473a2213db17ec472ed11b69056a as “Publish pinned Chatterbox Multilingual V3 GGUFs” and contains the V3 assets used by Subtitle Edit
• current cstr/chatterbox-turbo-GGUF snapshot resolves to revision b544cf8b49504d880640864a757b3ff3e4421a42 and contains the Turbo Q8_0 assets used by Subtitle Edit
• no SHA-256 claim is made for these Hugging Face/Xet model blobs; this PR establishes immutable repository revision identity and atomic local publication
• initial CI run #34720170036 exposed a regression-test expectation bug: DownloadHelper retries and wraps repeated HTTP failures in InvalidOperationException
• the regression was corrected to assert that public contract and preserve the original HttpRequestException as the inner cause
• full solution restore passed after the correction
• full solution build passed after the correction
• full solution test suite passed on the first run after the correction
• retry step was not used on the corrected run
• validation run: #34720966219

Base is current upstream fa6d8623c2adb0c646b6d31a11b221b855339bbe.

AI assistance: ChatGPT was used to audit the Chatterbox model provenance/publication path, verify immutable Hugging Face revisions, prepare the minimal pin/atomic-publication fix, and add regression coverage.

[Blackspirits]

Independent adversarial re-check completed after the regression-test correction. The Base and Turbo model URLs are pinned to verified immutable Hugging Face revisions c45504bb8d55473a2213db17ec472ed11b69056a and b544cf8b49504d880640864a757b3ff3e4421a42; model selection/file names remain unchanged; downloads stage to .part, preserve an existing destination until commit, re-check cancellation immediately before publish, and remove temporary files on failure. The earlier CI failure was a test expectation mismatch with DownloadHelper's documented retry/wrapping behavior, not a production defect. Corrected CI run #34720966219 passed restore/build/full tests on the first run with no retry. No blocker identified. Keep draft; no merge/promotion performed.