docs: def register_class(self, cls, app_state=None):
          """Register a model class."""
          state = app_state if app_state is not None else self.app_state
  
          for Adapter in self.adapters_cls:
              if Adapter.can_adapt(cls):
                  break
          else:
              return
  
          cls_fqcn = fqcn(cls)
          self.adapted[cls_fqcn] = Adapter(cls, self.schemas["default"])
          state.indexed_classes.add(cls)
          state.indexed_fqcn.add(cls_fqcn)