ENG-8388: refetch cached state when missing substates (#5991)

Avoid issue where hydrate operates on a subset of the tree because the Root
state was requested, but it didn't have all its substates cached, so it doesn't
return the full dict.

Author: masenf

reflex/istate/manager/redis.py

@@ -557,7 +557,12 @@ async def _get_state_cached(self, token: str) -> AsyncIterator[BaseState | None]
                     ) is not None:
                         # Make sure we have the substate cached (or fetch it from redis).
                         try:
-                            _ = cached_state.get_substate(state_path.split("."))
+                            substate = cached_state.get_substate(state_path.split("."))
+                            if len(substate.substates) != len(
+                                type(substate).get_substates()
+                            ):
+                                # If the substate is missing substates, we need to refetch it.
+                                raise ValueError  # noqa: TRY301
                         except ValueError:
                             await self.get_state(token, for_state_instance=cached_state)
                         yield cached_state