[Frontend] Move async logic outside of constructor (#4674)

tests/entrypoints/openai/test_serving_chat.py

@@ -20,11 +20,15 @@ class MockModelConfig:
 class MockEngine:
 
     async def get_model_config(self):
-        return MockModelConfig
+        return MockModelConfig()
 
 
 async def _async_serving_chat_init():
-    serving_completion = OpenAIServingChat(MockEngine(),
+    engine = MockEngine()
+    model_config = await engine.get_model_config()
+
+    serving_completion = OpenAIServingChat(engine,
+                                           model_config,
                                            served_model_names=[MODEL_NAME],
                                            response_role="assistant",
                                            chat_template=CHAT_TEMPLATE)