refactor(server): steward 决策链路改用 LangGraph 编排

- 新增 StewardGraphPlannerService，用 LangGraph 状态图编排意图识别→流程判断→模型/规则分支→兜底，替代原 planner 内线性调用
- 新增 StewardGraphRuntimeService 编排运行时决策与槽位决策；StewardActionContracts/Executor 统一动作合约与执行
- steward_intent_agent/application_fact_resolver/runtime_chat 适配图执行器，config 暴露图相关开关
- pyproject/uv.lock 新增 langgraph 依赖
- 新增 graph_planner/graph_runtime/action_executor 测试，更新 intent_agent/planner/fact_resolver/runtime_chat/reimbursement 测试

server/pyproject.toml

@@ -20,6 +20,7 @@ dependencies = [
   "email-validator>=2.2.0,<3.0.0",
   "python-multipart>=0.0.20,<1.0.0",
   "jieba>=0.42.1,<0.43.0",
+  "langgraph>=1.2.0,<2.0.0",
   "openpyxl>=3.1.5,<4.0.0",
   "lightrag-hku>=1.4.16,<1.5.0",
   "qdrant-client>=1.18.0,<2.0.0",