test: harden plot_unit_effects against ndim>0 scalar deprecation

Promote the numpy/xarray DeprecationWarning to an error so the
regression is caught on all Python versions, not just 3.14.

Author: anevolbap

causalpy/tests/test_panel_regression.py

@@ -290,6 +290,9 @@ def test_panel_regression_plot_coefficients(mock_pymc_sample, small_panel_data):
 
 
 @pytest.mark.integration
+@pytest.mark.filterwarnings(
+    "error:Conversion of an array with ndim > 0 to a scalar:DeprecationWarning"
+)
 def test_panel_regression_plot_unit_effects(mock_pymc_sample, small_panel_data):
     """Test plot_unit_effects method."""
     result = cp.PanelRegression(