🧪 scripts: add missing tests for _os_func error handling in dl_github_archive.py

[ManupaKDU]

🎯 What: Missing test coverage for the error handling logic in scripts/dl_github_archive.py:100 (Path._os_func), where it swallows expected OS errors (by matching errno) and re-raises others.
📊 Coverage: Added test cases for successful execution, expected OSError (returns default), unexpected OSError (re-raises), and other unexpected exceptions (re-raises).
✨ Result: Improved test coverage and guaranteed robustness of the helper function that handles filesystem interaction in the download script.

Signed-off-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

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PR created automatically by Jules for task 8044335736364615757 started by @manupawickramasinghe

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