Explicit root port count check

Co-authored-by: Douglas Wilson <141026920+doug-q@users.noreply.github.com>

hugr-core/src/hugr/views/sibling.rs

@@ -501,6 +501,7 @@ mod test {
 
         assert_eq!(region.node_count(), extracted.node_count());
         assert_eq!(region.root_type(), extracted.root_type());
+        assert_eq!(0, extracted.input_count(extracted.root()) + extracted.output_count(extracted.root()));
 
         Ok(())
     }