Fix: correct misplaced `ensure_ascii=False` (#22997)

Signed-off-by: Yongtao Huang <yongtaoh2022@gmail.com>

api/core/workflow/nodes/question_classifier/question_classifier_node.py

                 text=QUESTION_CLASSIFIER_COMPLETION_PROMPT.format(
                     histories=memory_str,
                     input_text=input_text,
-                    categories=json.dumps(categories),
+                    categories=json.dumps(categories, ensure_ascii=False),
                     classification_instructions=instruction,
-                    ensure_ascii=False,
                 )
             )