Fix mixed-type categorical sorting crashes (fixes #273)

[oegedijk]

Summary

• fix issue Categorical columns #273 by hardening categorical sorting against mixed types and missing values
• add a shared sorted_categorical_values(...) helper and reuse it across explainer setup, categorical ordering, PDP grid generation, and categorical plot code paths
• harden plotly_shap_violin_plot for non-string category labels
• add regression tests for mixed categorical values in plots and mixed target-label fallback sorting
• update release notes and move TODO Now to the next item

Files touched

• explainerdashboard/explainer_methods.py
• explainerdashboard/explainers.py
• explainerdashboard/explainer_plots.py
• tests/test_datasets.py
• tests/test_categorical_sorting.py
• RELEASE_NOTES.md
• TODO.md

Validation

• uv run pytest tests/test_datasets.py tests/test_categorical_sorting.py -q
  • result: 9 passed

Fixes #273