reject broken symlinks instead of skipping them as cycles

[Support-itecz]

Summary

check_symlink() (added in #680) treats every failure of path.resolve(strict=True) as a symlink cycle: it logs "Symlink cycle detected" and both serializers skip the entry. A dangling symlink fails strict resolution too, so with allow_symlinks enabled it is silently left out of the manifest when signing, and one added to a signed model afterwards passes verification unreported, while an added regular file fails with "Extra files found in model". Since #667 the verifier applies the allow_symlinks policy recorded in the signature, so any model signed with symlinks allowed verifies this way regardless of the caller's flag. The link can name a target that only appears after verification (under /tmp, a cache path), at which point a loader walking the verified directory reads content nobody signed. Before #680 a broken symlink failed closed with a ValueError from check_file_or_directory.

Narrow the cycle handling to actual loops (RuntimeError on Python 3.12 and earlier, OSError with ELOOP on 3.13 and later) and raise ValueError for a symlink that cannot be resolved, at both signing and verification. To reproduce: sign a directory with set_allow_symlinks(True), add (model / "x").symlink_to(model / "missing"), verify. Before this change verification succeeds; with it, it fails the same way an added regular file does.

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