Fix mypy union-attr errors in PanelRegression

Existing type: ignore[attr-defined] comments did not cover the
union-attr error that mypy now flags on self.model.idata.posterior
(self.model.idata is Any | None).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: cetagostini

causalpy/experiments/panel_regression.py

@@ -557,7 +557,7 @@ def get_plot_data_bayesian(self, **kwargs: Any) -> pd.DataFrame:
         """
         # Get posterior predictions
         if isinstance(self.model, PyMCModel):
-            mu = self.model.idata.posterior["mu"]  # type: ignore[attr-defined]
+            mu = self.model.idata.posterior["mu"]  # type: ignore[attr-defined,union-attr]
             pred_mean = mu.mean(dim=["chain", "draw"]).values.flatten()
             pred_lower = mu.quantile(0.025, dim=["chain", "draw"]).values.flatten()
             pred_upper = mu.quantile(0.975, dim=["chain", "draw"]).values.flatten()
@@ -673,7 +673,7 @@ def plot_unit_effects(
 
         if isinstance(self.model, PyMCModel):
             # Bayesian: get posterior means
-            beta = self.model.idata.posterior["beta"]  # type: ignore[attr-defined]
+            beta = self.model.idata.posterior["beta"]  # type: ignore[attr-defined,union-attr]
             unit_fe_indices = [self.labels.index(name) for name in unit_fe_names]
 
             # Get mean and std for each unit FE