feat: 实现 React 模式循环推理，带超时重试和结构化错误处理

- 更新 intent.py 为 React 模式推理器
- 新增 react_nodes.py: React 模式节点
- 新增 retry_utils.py: 超时和重试工具
- 更新 state.py: 支持循环步数和错误记录
- 重写 subgraph_builder.py: 完整 React 循环流程
- 结构化错误输出，符合 Agent 执行循环最佳实践
- 限制最大推理步数 ≤40，防止无限循环
- RAG 检索带重试和超时保护
- 子图错误可传递给主图处理

backend/app/graph/__init__.py

@@ -1,21 +1,63 @@
 """
-Graph 子模块
+Graph 子模块 - React 模式增强版（带超时重试）
 """
 
 from .graph_builder import GraphBuilder
-from .subgraph_builder import build_main_graph
+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 .state import (
     MessagesState,
     GraphContext,
     MainGraphState,
-    CurrentAction
+    CurrentAction,
+    ErrorRecord,
+    ErrorSeverity
+)
+from .retry_utils import (
+    RetryConfig,
+    RetryResult,
+    RetryStrategy,
+    with_retry,
+    with_async_retry,
+    create_retry_wrapper_for_node,
+    RAG_RETRY_CONFIG,
+    SUBGRAPH_RETRY_CONFIG
 )
 
 __all__ = [
+    # 旧版兼容性
     "GraphBuilder",
     "build_main_graph",
     "MessagesState",
     "GraphContext",
     "MainGraphState",
-    "CurrentAction"
+    "CurrentAction",
+    
+    # 新版 React 模式
+    "build_react_main_graph",
+    "init_state_node",
+    "react_reason_node",
+    "rag_retrieve_node",
+    "error_handling_node",
+    "final_response_node",
+    "route_by_reasoning",
+    "ErrorRecord",
+    "ErrorSeverity",
+    
+    # 超时和重试工具
+    "RetryConfig",
+    "RetryResult",
+    "RetryStrategy",
+    "with_retry",
+    "with_async_retry",
+    "create_retry_wrapper_for_node",
+    "RAG_RETRY_CONFIG",
+    "SUBGRAPH_RETRY_CONFIG"
 ]