Fix mask geometry to ensure consistent MultiPolygon output across Python versions

Author: paulnoirel

libs/labelbox/src/labelbox/data/annotation_types/geometry/mask.py

@@ -62,7 +62,13 @@ def geometry(self) -> Dict[str, Tuple[int, int, int]]:
         if not holes.is_valid:
             holes = holes.buffer(0)
 
-        return external_polygons.difference(holes).__geo_interface__
+        result = external_polygons.difference(holes)
+        
+        # Ensure consistent MultiPolygon output across Python versions
+        if hasattr(result, "geom_type") and result.geom_type == "Polygon":
+            result = MultiPolygon([result])
+        
+        return result.__geo_interface__
 
     def draw(
         self,