fix: update test import to match renamed LLM service class

Author: shinersup

backend/tests/test_review.py

@@ -236,7 +236,7 @@ def test_cache_key_differs_for_different_action(self):
     @pytest.mark.asyncio
     async def test_cache_hit_skips_llm_call(self):
         """When cache has a result, the LLM should NOT be called."""
-        from app.services.llm import LLMService
+        from app.services.llm import MockLLMService as LLMService
 
         service = LLMService.__new__(LLMService)
         service.review_parser = MagicMock()