恢复循环推理架构，子图执行完回到react_reason

- 恢复子图→react_reason的边
- 保持intent.py中的逻辑：检测到subgraph_completed就返回DIRECT_RESPONSE
- 保持llm_call中的逻辑：检测到final_result就直接返回

backend/app/main_graph/utils/main_graph_builder.py

@@ -235,16 +235,14 @@ def build_react_main_graph(llm=None, tools=None, mem0_client=None) -> StateGraph
         }
     )
 
-    # 循环边（rag、web_search、error 回到 reason）
+    # 循环边（rag、web_search、子图、error都回到reason）
     graph.add_edge("rag_retrieve", "react_reason")
     graph.add_edge("web_search", "react_reason")
+    graph.add_edge("contact_subgraph", "react_reason")
+    graph.add_edge("dictionary_subgraph", "react_reason")
+    graph.add_edge("news_analysis_subgraph", "react_reason")
     graph.add_edge("handle_error", "react_reason")
     
-    # 关键修改：子图执行完后直接去 finalize（避免循环）
-    graph.add_edge("contact_subgraph", "finalize")
-    graph.add_edge("dictionary_subgraph", "finalize")
-    graph.add_edge("news_analysis_subgraph", "finalize")
-    
     # 第三阶段：llm_call 后进入完成处理
     if llm_node is not None:
         if summarize_node: