Handle cross-mount paths in find_parents

[lin-hongkuan]

find_parents() uses os.path.commonprefix() as a guard before calling os.path.relpath(), but commonprefix() is character-based. On Windows UNC paths such as \\server\share1 and \\server\share2\path.py, the prefix check passes even though the paths are on different mounts, and relpath() raises ValueError.

This catches that ValueError and returns [], matching the existing behavior for paths outside the root. The regression test uses ntpath directly so it covers the Windows UNC behavior on any platform.

fixes #705

Tests:

• python -m pytest -o addopts="" test/test_utils.py::test_find_parents_handles_cross_mount_paths test/test_utils.py::test_find_parents -q
• python -m pytest -o addopts="" test/test_utils.py -q
• py -3 -m ruff check pylsp/_utils.py test/test_utils.py