docs: def discover_extensions(self):
          """Discover available extensions."""
          if self._discovery_done:
              return
  
          try:
              previous_state = self._extensions.copy()
  
              for app_config in apps.get_app_configs():
                  indexes_path = '{}.extensions'.format(app_config.name)
                  try:
                      import_module(indexes_path)
                  except ImportError:
                      pass
  
              self._discovery_done = True
          except Exception:
              # Rollback state to prevent corrupted state on exceptions during import.
              self._extensions = previous_state
              raise