Bonsai: plain-language message for Extend To Underside guard (#7454)

Per the reporter's follow-up on #7454, replace the LAYER2 jargon error
with a message that names walls as the supported selection and states
that extending columns or similar elements is not implemented yet. Also
drop the explanatory inline comment block per project comment standards;
the guard exists so an invalid selection (e.g. column plus slab with no
wall) reports a clean error instead of re-raising the nested operator's
CANCELLED as a Python traceback.

Refs #7454

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/model/workspace.py

@@ -1280,7 +1280,20 @@ class Hotkey(bpy.types.Operator, tool.Ifc.Operator):
             bpy.ops.bim.extend_profile(join_type="T")
 
         else:
-            bpy.ops.bim.extend_walls_to_underside()
+            walls = [
+                obj
+                for obj in bpy.context.selected_objects
+                if (element := tool.Ifc.get_entity(obj)) and tool.Parametric.is_path_connectable_wall(element)
+            ]
+            if walls and len(bpy.context.selected_objects) > len(walls):
+                bpy.ops.bim.extend_walls_to_underside()
+            else:
+                self.report(
+                    {"ERROR"},
+                    "Extend to underside works with layered walls (LAYER2): "
+                    "select at least one wall plus the target element. "
+                    "Extending columns or similar elements is not supported yet.",
+                )
 
     def hotkey_S_F(self):
         if not bpy.context.selected_objects: