feat(llm): migrate model calls to LangChain ChatOpenAI

test/agent/test_llm_error_mapping.py

@@ -37,9 +37,7 @@ def _service_with_failing_llm(exc: Exception, max_retries: int = 0):
     """complete 固定抛 exc 的 OpenAILLMService（client 打桩）。"""
     llm = OpenAILLMService(model="test", api_key="test", max_retries=max_retries)
     create = AsyncMock(side_effect=exc)
-    llm._client = SimpleNamespace(
-        chat=SimpleNamespace(completions=SimpleNamespace(create=create))
-    )
+    llm._client = SimpleNamespace(ainvoke=create)
     return llm, create