Auto-infer mask contour thickness and consolidate tests folder

[clemsgrs]

Summary

• make tissue/hole mask contour thickness fully automatic based on level-0 WSI dimensions and visualization scale
• remove manual line_thickness argument from WholeSlideImage.visualize_mask
• add targeted tests for adaptive thickness behavior and API breakage
• consolidate legacy test/ into a single tests/ folder (including fixtures/helpers)
• update pytest discovery and CI workflow to use tests/

Testing

• /Users/clems/Code/venv/hs2p/bin/python -m pytest /Users/clems/Code/hs2p/tests -q
• /Users/clems/Code/venv/hs2p/bin/python -m pytest /Users/clems/Code/hs2p/tests/test_visualize_mask_thickness.py -q
• /Users/clems/Code/venv/hs2p/bin/python -m pytest /Users/clems/Code/hs2p/tests/test_real_fixture_smoke_regression.py -q