优化：重构 state，分离持久化和临时字段，完善 init_state_node

主要改动：
1. 移除无用字段 system_prompt
2. 重新组织 state，明确标注「持久化字段」和「临时字段」
3. 完善 init_state_node，重置所有临时字段
4. 解决数据残留隐患，确保每轮对话开始时状态干净

backend/app/main_graph/nodes/routing.py

@@ -11,21 +11,97 @@
 from datetime import datetime
 
 from backend.app.core.intent import get_route_by_reasoning, ReasoningAction
-from ...main_graph.state import MainGraphState
+from ...main_graph.state import (
+    MainGraphState,
+    CurrentAction,
+    ReactReasoningState,
+    HybridRouterState,
+    FastPathState
+)
 from backend.app.logger import info
 
 
 # ========== 初始化状态节点 ==========
 def init_state_node(state: MainGraphState) -> MainGraphState:
-    """初始化状态节点：在流程开始时设置初始值"""
+    """
+    初始化状态节点：在流程开始时设置初始值
+    
+    重置策略：
+    - 持久化字段（如 messages、turns_since_last_summary）不重置
+    - 临时字段（如 rag_context、final_result）重置为初始值
+    """
+    # 持久化字段保留原样
+    # - messages
+    # - turns_since_last_summary
+    # - user_id
+    
+    # ========== 重置临时字段 ==========
+    
+    # 主图控制字段
+    state.user_query = ""
+    state.current_action = CurrentAction.NONE
+    state.current_model = ""
+    state.intent_confidence = 0.0
+    
+    # React 推理专用字段
+    state.reasoning_step = 0
+    state.last_action = ""
+    state.reasoning_history = []
+    
+    # RAG 相关字段
+    state.rag_context = ""
+    state.rag_retrieved = False
+    state.rag_docs = []
+    state.rag_confidence = 0.0
+    state.rag_attempts = 0
+    
+    # 联网搜索相关字段
+    state.web_search_results = []
+    
+    # 错误处理字段
+    state.errors = []
+    state.current_error = None
+    state.retry_action = None
+    state.error_message = ""
+    
+    # 子图结果字段
+    state.news_result = None
+    state.dictionary_result = None
+    state.contact_result = None
+    
+    # 执行状态
+    state.current_phase = "initializing"
+    state.final_result = ""
+    state.success = False
+    
+    # 元数据
+    state.start_time = None
+    state.end_time = None
+    
+    # 结构化状态
+    state.react_reasoning = ReactReasoningState()
+    state.hybrid_router = HybridRouterState()
+    state.fast_path = FastPathState()
+    
+    # 统计字段
+    state.llm_calls = 0
+    state.last_token_usage = {}
+    state.last_elapsed_time = 0.0
+    state.memory_context = ""
+    
+    # 向后兼容字段
+    state.debug_info = {}
+    
+    # 设置初始值
     state.current_phase = "initializing"
     state.reasoning_step = 0
     state.start_time = datetime.now().isoformat()
-
+    
+    # 从 messages 中提取 user_query（如果没有的话）
     if not state.user_query and state.messages:
         last_msg = state.messages[-1]
         state.user_query = getattr(last_msg, "content", str(last_msg))
-
+    
     return state