Raise on missing string/sensor labels in ExamplePrometheusGridDefinition

Mirror the IC86 grid's validation: initialize the string and sensor-number
indices to None and raise a ValueError if either label is absent from the
pixel feature names, instead of leaving the attributes unset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: sevmag

src/graphnet/models/data_representation/images/mappings/grid_definition.py

@@ -419,6 +419,8 @@ def _set_indices(
     ) -> None:
         """Set the indices for the features."""
         self._cnn_features_idx = []
+        self._sensor_number_idx = None
+        self._string_idx = None
         for feature in feature_names:
             if feature == sensor_number_label:
                 self._sensor_number_idx = feature_names.index(feature)
@@ -427,6 +429,16 @@ def _set_indices(
             else:
                 self._cnn_features_idx.append(feature_names.index(feature))
 
+        if self._sensor_number_idx is None:
+            raise ValueError(
+                f"Sensor number label not found in feature names: "
+                f"{sensor_number_label}"
+            )
+        if self._string_idx is None:
+            raise ValueError(
+                f"String label not found in feature names: {string_label}"
+            )
+
     def forward(self, data: Data, data_feature_names: List[str]) -> Data:
         """Scatter pixel rows into the example 3D image tensor."""
         # Initialize output arrays