♻️ Testing suite refactor + model changes

[azogue]

Maintenance-only update, without new features.

Changes

• 🎨 lint: pre-commit autoupdate
• 📦️ env: Add slugify to deps and bump version
• 🐛 Ensure curve arrays have valid shapes
• ✅ tests: Add more tests for sat lines crossing axes and detail for wbt slope change zone
• ✅ tests: Add more unit tests to reach full coverage
• ⚡️ tests: Minor optimization to run less PNG exports (or with < dpi), and remove unnecessary calls to chart.close_fig()
• 🔥 tests: Remove unnecessary
• 💥 Add 'internal_value' field for each PsychroCurve with better validation of data arrays
• 💥 Remove plotting methods from PsychroCurve/PsychroCurves for a more functional approach using methods in plot_logic.py