Fix #124484: Debug assert on undoing out of Sculpt Mode

While this is a rather fragile fix, the alternative also seems fragile.
We can instead check the operator name inside `push_begin_ex` to see if
the operator is transitioning into Sculpt Mode and initialize the
StepData.type to a custom "Mode Change" type instead of using "None"

Pull Request: https://projects.blender.org/blender/blender/pulls/124640

source/blender/editors/sculpt_paint/sculpt_undo.cc

@@ -911,6 +911,12 @@ static void restore_list(bContext *C, Depsgraph *depsgraph, StepData &step_data)
     return;
   }
 
+  /* Switching to sculpt mode does not push a particular type.
+   * See #124484. */
+  if (step_data.type == Type::None && step_data.nodes.is_empty()) {
+    return;
+  }
+
   const bool tag_update = ID_REAL_USERS(object.data) > 1 ||
                           !BKE_sculptsession_use_pbvh_draw(&object, rv3d) || ss.shapekey_active ||
                           ss.deform_modifiers_active;