Fix UpdatedVariable truncation crash (langgenius#27359)

Signed-off-by: -LAN- <laipz8200@outlook.com>

Author: laipz8200

api/services/variable_truncator.py

@@ -283,7 +283,7 @@ def _truncate_array(self, value: list[object], target_size: int) -> _PartResult[
                 break
 
             remaining_budget = target_size - used_size
-            if item is None or isinstance(item, (str, list, dict, bool, int, float)):
+            if item is None or isinstance(item, (str, list, dict, bool, int, float, UpdatedVariable)):
                 part_result = self._truncate_json_primitives(item, remaining_budget)
             else:
                 raise UnknownTypeError(f"got unknown type {type(item)} in array truncation")
@@ -373,6 +373,11 @@ def _truncate_object(self, mapping: Mapping[str, Any], target_size: int) -> _Par
 
         return _PartResult(truncated_obj, used_size, truncated)
 
+    @overload
+    def _truncate_json_primitives(
+        self, val: UpdatedVariable, target_size: int
+    ) -> _PartResult[Mapping[str, object]]: ...
+
     @overload
     def _truncate_json_primitives(self, val: str, target_size: int) -> _PartResult[str]: ...