raise instead of skipping model directories that cannot be listed

[Support-itecz]

Summary

check_file_or_directory accepts any path that stats as a directory, but glob("**/*") drops a subtree it cannot list without surfacing the error.

• CPython swallows the failure inside the per-directory generator (PermissionError on 3.10-3.12, OSError in glob._Globber on 3.13+)
• the manifest records only files, so an unreadable directory contributes zero entries and nothing raises
• mkdir model/evil; cp backdoor.py model/evil/; chmod 0111 model/evil keeps backdoor.py out of the manifest and verify still reports success; mode 0111 still grants search, so whatever loads the model opens the planted file normally
• both the file and shard serializers call this helper, so one check covers both walk loops

The docstring already said this call raises when there are permission errors. Signing or verifying a tree you cannot fully read now fails instead of quietly covering less than it appears to.

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed