Fix prototype state user isolation

sdk/prototype_state.py

@@ -44,7 +44,7 @@ class PrototypeStateStore:
         if existing is not None:
             stored = existing.model_copy(update={"is_new": False})
             self._users[key] = stored
-            return stored
+            return stored.model_copy()
 
         user = User(
             user_id=f"usr-{platform}-{external_id}",
@@ -55,7 +55,7 @@ class PrototypeStateStore:
             is_new=True,
         )
         self._users[key] = user
-        return user
+        return user.model_copy()
 
     async def get_settings(self, user_id: str) -> UserSettings:
         stored = self._settings.get(user_id, {})