chore: improve platformdirs maintenance path (#488)

Summary:
- Add edge-case unit tests for MacOS path resolution (e.g.,
site_runtime_dir/site_runtime_path, multipath=True behavior,
ensure_exists=True with pre-existing dir) and tighten type annotations /
add explicit return type on any helper missing it in macos.py.
- Keep the change narrow so it is straightforward to review.

Notes:
- I kept this scoped to the relevant implementation and tests.

Co-authored-by: Noa Levi <275430404+lphuc2250gma@users.noreply.github.com>