Use ._is_mutable() to account for parent state proxy (#3739)

When a parent state proxy is set, also allow child StateProxy._self_mutable to
override the parent's `_is_mutable()`.

reflex/state.py

@@ -2059,7 +2059,7 @@ def _is_mutable(self) -> bool:
             Whether the state is mutable.
         """
         if self._self_parent_state_proxy is not None:
-            return self._self_parent_state_proxy._is_mutable()
+            return self._self_parent_state_proxy._is_mutable() or self._self_mutable
         return self._self_mutable
 
     async def __aenter__(self) -> StateProxy:
@@ -3302,7 +3302,7 @@ def _mark_dirty(
         Raises:
             ImmutableStateError: if the StateProxy is not mutable.
         """
-        if not self._self_state._self_mutable:
+        if not self._self_state._is_mutable():
             raise ImmutableStateError(
                 "Background task StateProxy is immutable outside of a context "
                 "manager. Use `async with self` to modify state."