chore: improve platformdirs maintenance path

[lphuc2250gma]

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.

[gaborbernat]

LGTM