[WIP] Fix errors with hf download command

[Copilot]

• Analyze the issue: huggingface_hub requires ETag header in HEAD responses for file downloads, but the mirror doesn't set it for HuggingFace cache responses
• Fix huggingface.go: Set ETag header wherever X-Repo-Commit is set (two locations: direct SHA match on line 41, and inside setFromCache closure on line 78)
• Add tests for the ETag header behavior in HuggingFace header handling
• Verify the fix builds and passes tests
• Run code review and CodeQL check

Original prompt

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This section details on the original issue you should resolve

<issue_title>Fixes with hf download </issue_title>
<issue_description>```
Traceback (most recent call last): | 0/10 [00:00<?, ?it/s]
File "/opt/homebrew/Cellar/huggingface-cli/1.4.1/libexec/lib/python3.14/site-packages/huggingface_hub/file_download.py", line 1736, in _get_metadata_or_catch_error
raise FileMetadataError(
"Distant resource does not have an ETag, we won't be able to reliably ensure reproducibility."
)
huggingface_hub.errors.FileMetadataError: Distant resource does not have an ETag, we won't be able to reliably ensure reproducibility.

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Fixes with hf download  #20

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