test: add guarded coverage for app.py dashboard SHAP helpers

[AmirhosseinHonardoust]

Adds the last missing test coverage: app.py's SHAP-path helpers.

• test_plot_single_shap_bar_returns_figure: unit test for the
  single-transaction SHAP bar chart.
• test_explain_single_transaction_end_to_end: fits a small pipeline,
  scores a batch, and asserts explain_single_transaction returns a
  matplotlib Figure plus non-empty reason codes (covers the PR for
  modern-shap compatibility in app.py's actual call path).

Both tests skip cleanly when streamlit/shap are unavailable and run in
CI where requirements.txt installs them.

Verified locally: ruff/black/mypy clean; 41/41 tests pass with deps;
the 2 new tests skip cleanly without them.