fix: avoid zero-inode samefile matches on network filesystems

[aryansk]

Fixes #14864

Some network filesystems report st_ino == 0 for every path. samefile_nofollow() now avoids calling samestat() when either inode is zero, preventing unrelated paths from being treated as the same file. Added regression coverage for all zero-inode combinations.

Testing:

• PYTHONPATH=src uv run --no-project --python 3.13 --with pytest --with pluggy --with iniconfig --with packaging python -m pytest -q testing/test_pathlib.py -k "samefile_nofollow or samefile_false_negatives"

Prepared with AI assistance; maintainer review is requested.

[RonnyPfannschmidt]

duplicate of #14865

[aryansk]

Thanks for pointing me to #14865. I have not reopened or duplicated the fix; this draft is closed and the issue is now covered by the maintainer PR.