refactor: 将 RAG 节点拆分为独立模块

- 新增 rag_nodes.py: 独立的 RAG 检索节点
- 从 react_nodes.py 移除 RAG 相关代码
- 更新导入和导出
- rag_nodes.py 包含 rag_retrieve_node 和 rag_re_retrieve_node
- 添加 inject_rag_tool_to_state 工具函数

backend/app/graph/__init__.py

@@ -7,11 +7,16 @@ from .subgraph_builder import build_main_graph, build_react_main_graph
 from .react_nodes import (
     init_state_node,
     react_reason_node,
-    rag_retrieve_node,
     error_handling_node,
     final_response_node,
     route_by_reasoning
 )
+from .rag_nodes import (
+    rag_retrieve_node,
+    rag_re_retrieve_node,
+    inject_rag_tool_to_state,
+    get_rag_tool_from_state
+)
 from .state import (
     MessagesState,
     GraphContext,
@@ -44,13 +49,18 @@ __all__ = [
     "build_react_main_graph",
     "init_state_node",
     "react_reason_node",
-    "rag_retrieve_node",
     "error_handling_node",
     "final_response_node",
     "route_by_reasoning",
     "ErrorRecord",
     "ErrorSeverity",
     
+    # RAG 节点（独立模块）
+    "rag_retrieve_node",
+    "rag_re_retrieve_node",
+    "inject_rag_tool_to_state",
+    "get_rag_tool_from_state",
+    
     # 超时和重试工具
     "RetryConfig",
     "RetryResult",