test(domain): assert IllegalTransition Display message

crates/domain/src/object.rs

@@ -180,6 +180,11 @@ mod tests {
                 to: Draft
             })
         );
+        // the Display message is the user-visible surface of the error
+        assert_eq!(
+            Draft.transition_to(Public).unwrap_err().to_string(),
+            "illegal visibility transition: draft -> public"
+        );
     }
 
     #[test]