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/subgraph_builder.py

@@ -10,11 +10,11 @@ from .state import MainGraphState, CurrentAction
 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
 from ..agent_subgraphs.contact import build_contact_subgraph
 from ..agent_subgraphs.dictionary import build_dictionary_subgraph
 from ..agent_subgraphs.news_analysis import build_news_analysis_subgraph