chore(backend): 放宽递归与循环检测阈值

- build_run_config recursion_limit 100 -> 250
- LoopDetectionMiddleware 默认阈值放宽（warn/hard/window/tool-freq）

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/app/gateway/services.py

@@ -314,7 +314,7 @@ def build_run_config(
     the LangGraph Platform-compatible HTTP API and the IM channel path behave
     identically.
     """
-    config: dict[str, Any] = {"recursion_limit": 100}
+    config: dict[str, Any] = {"recursion_limit": 250}
     if request_config:
         # LangGraph >= 0.6.0 introduced ``context`` as the preferred way to
         # pass thread-level data and rejects requests that include both