fix: restore macOS 15 build matrix and correct test path

- Add back macOS 15 configurations for Python 3.9-3.13
- Fix pytest path from test/ to tests/ (correct directory name)

The macOS 15 support was accidentally missing from the matrix, and
pytest was looking for the wrong directory name.

.github/workflows/build-reusable.yml

@@ -289,7 +289,7 @@ jobs:
           source .venv/bin/activate || source .venv/Scripts/activate
 
           # Run tests
-          pytest -v test/
+          pytest -v tests/
 
       - name: Run sanity checks (optional)
         run: |