优化：复用 hybrid_router 的推理结果，避免重复 LLM 调用

backend/app/main_graph/nodes/hybrid_router.py

@@ -30,6 +30,7 @@ class HybridRouterResult:
     suggested_tools: list = field(default_factory=list)
     path: str = "react_loop"  # fast_chitchat / fast_rag / fast_tool / react_loop
     reasoning: str = ""
+    reasoning_result: Optional[ReasoningResult] = None  # 保存完整的 ReasoningResult，用于复用！
 
 
 # ========== 规则配置 ==========
@@ -87,7 +88,8 @@ def _map_reasoning_to_router(reasoning_result: ReasoningResult) -> HybridRouterR
         confidence=reasoning_result.confidence,
         suggested_tools=suggested_tools,
         path=path,
-        reasoning=reasoning_result.reasoning
+        reasoning=reasoning_result.reasoning,
+        reasoning_result=reasoning_result  # 保存完整结果！
     )