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/tests/test_reimbursement_endpoints.py

@@ -939,7 +939,7 @@ def test_application_preview_action_submits_without_orchestrator_run(monkeypatch
     assert draft_payload["draft_type"] == "expense_application"
     assert draft_payload["status"] == "submitted"
     assert draft_payload["approval_stage"] == "直属领导审批"
-    assert draft_payload["claim_no"].startswith("AP-")
+    assert draft_payload["claim_no"].startswith("A")
 
     with session_factory() as db:
         claim = db.get(ExpenseClaim, draft_payload["claim_id"])
@@ -1015,7 +1015,7 @@ def test_application_preview_action_saves_draft_with_detail_reference(monkeypatc
     assert draft_payload["status"] == "draft"
     assert draft_payload["approval_stage"] == "待提交"
     assert draft_payload["claim_id"]
-    assert draft_payload["claim_no"].startswith("AP-")
+    assert draft_payload["claim_no"].startswith("A")
 
     with session_factory() as db:
         claim = db.get(ExpenseClaim, draft_payload["claim_id"])