Replace string prefix matching with Path.is_relative_to() to correctly
handle Windows paths like "C:\" where trailing backslash caused double
separator issues (e.g., "C:\\" instead of "C:\").
Changes:
- Use Path.is_relative_to() for subdirectory detection (requires Python 3.9+)
- Add Windows path handling tests using PureWindowsPath
- Update test_utils.py to expect /etc/passwd to be blocked (security fix)
91ffb51564