fix: restore llm model validation dependency

tests/test_llm_gateway.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
 import unittest
+from unittest.mock import patch
 
 from backend.llm import LLMGateway, LLMGatewayError
+from backend.llm.service import LLMServiceMixin
 
 
 class ProviderFailure(RuntimeError):
@@ -113,6 +115,31 @@ class LLMGatewayTests(unittest.TestCase):
         with self.assertRaisesRegex(LLMGatewayError, "额度已用完"):
             gateway.call("mentor", "mentor-v1", lambda model: "unused", (ProviderFailure,))
 
+    def test_saved_system_model_can_reach_the_connection_probe(self) -> None:
+        service = LLMServiceMixin()
+        service._system_credentials = {
+            "llm_models": [
+                {
+                    "id": "primary-model",
+                    "name": "主模型",
+                    "api_key": "secret",
+                    "base_url": "https://model.example/v1",
+                    "model": "model-name",
+                }
+            ]
+        }
+        service.llm_gateway = self.gateway()
+        expected = {"ok": True, "reply": "OK"}
+        with patch(
+            "backend.llm.service.test_llm_connection", return_value=expected
+        ) as connection_probe:
+            result = service.test_system_llm_profile("primary-model", {})
+
+        self.assertEqual(result, expected)
+        connection_probe.assert_called_once_with(
+            "secret", "https://model.example/v1", "model-name"
+        )
+
 
 if __name__ == "__main__":
     unittest.main()