improve state hierarchy validation, drop old testing special case (#3894)

reflex/app.py

@@ -269,13 +269,12 @@ class App(MiddlewareMixin, LifespanMixin, Base):
                 "`connect_error_component` is deprecated, use `overlay_component` instead"
             )
         super().__init__(**kwargs)
-        base_state_subclasses = BaseState.__subclasses__()
 
         # Special case to allow test cases have multiple subclasses of rx.BaseState.
-        if not is_testing_env() and len(base_state_subclasses) > 1:
-            # Only one Base State class is allowed.
+        if not is_testing_env() and BaseState.__subclasses__() != [State]:
+            # Only rx.State is allowed as Base State subclass.
             raise ValueError(
-                "rx.BaseState cannot be subclassed multiple times. use rx.State instead"
+                "rx.BaseState cannot be subclassed directly. Use rx.State instead"
             )
 
         if "breakpoints" in self.style:

reflex/state.py

@@ -495,21 +495,11 @@ class BaseState(Base, ABC, extra=pydantic.Extra.allow):
             if cls.get_name() in set(
                 c.get_name() for c in parent_state.class_subclasses
             ):
-                if is_testing_env():
-                    # Clear existing subclass with same name when app is reloaded via
-                    # utils.prerequisites.get_app(reload=True)
-                    parent_state.class_subclasses = set(
-                        c
-                        for c in parent_state.class_subclasses
-                        if c.get_name() != cls.get_name()
-                    )
-                else:
-                    # During normal operation, subclasses cannot have the same name, even if they are
-                    # defined in different modules.
-                    raise StateValueError(
-                        f"The substate class '{cls.get_name()}' has been defined multiple times. "
-                        "Shadowing substate classes is not allowed."
-                    )
+                # This should not happen, since we have added module prefix to state names in #3214
+                raise StateValueError(
+                    f"The substate class '{cls.get_name()}' has been defined multiple times. "
+                    "Shadowing substate classes is not allowed."
+                )
             # Track this new subclass in the parent state's subclasses set.
             parent_state.class_subclasses.add(cls)