Add breaking changes check to test_ancestor_filtering_with_mixed_changes

Verify that adding a non-nullable struct correctly reports as a breaking change
in the ancestor filtering test with mixed changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: murali-db

kernel/src/schema/diff.rs

@@ -1202,6 +1202,9 @@ mod tests {
         assert!(paths.contains(&ColumnName::new(["existing_struct"])));
         assert!(paths.contains(&ColumnName::new(["existing_struct", "new_name"])));
 
+        // Added a non-nullable struct "new_struct"
+        assert!(diff.has_breaking_changes());
+
         // nested_field should NOT appear as added since new_struct is its ancestor
     }