Fix cleanup of empty directories with read-only parents - #15360

Summary

When pytest removes a session directory containing an empty directory whose parent is read-only, the os.rmdir retry can fail again because only the leaf was made writable. That error is swallowed by the numbered-directory cleanup, leaving the entire session tree as a garbage-* directory. Restore permissions on parent directories before retrying, and keep a stubborn entry from aborting the rest of the removal.

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 python -m pytest testing/test_tmpdir.py -q (57 passed, 12 skipped)
• python -m ruff check src/_pytest/pathlib.py testing/test_tmpdir.py
• git diff --check

Fixes #15341