Rendering any admin page stamps the sidebar's managed-models set onto the registered `AdminManageNavGroupAction` singleton (inside `AdminSidebarManageActionGroupRenderer._get_sidebar_state()`), and that instance attribute survived across tests. Once any earlier test rendered admin chrome, the "unmanaged" cache tests saw User as managed, the post_save/post_delete handlers invalidated the cache, and the tests failed. The test class now clears the stamped state from the registered action in `setUp()`, and `_patch_managed_models()` patches the registered instance instead of the class, which the instance attribute would otherwise shadow. Testing Done: Ran unit tests in random order.