Network failure-proof the pull tests

[rasapala]

🛠 Summary

Making pull cache tests network failure proof.

🧪 Checklist

• Unit tests added.
• The documentation updated.
• Change follows security best practices.
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[Copilot]

Pull request overview

Improves robustness of the HuggingFace pull-cache test fixture to better tolerate transient network/LFS failures when initializing the shared cached repository used by HfPullCache tests.

Changes:

• Adds a helper to run --pull synchronously and capture the server exit code.
• Adds retry logic (max attempts + delay) and cache integrity checks (presence + expected file sizes + absence of .lfswip markers) when initializing the shared HF cache.
• Seeds each test’s temp repository by copying from the shared cached repository.