[Bugfix] Fix AttributeError: 'State' object has no attribute 'engine_client' (#17434)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

vllm/entrypoints/api_server.py

@@ -111,7 +111,7 @@ async def init_app(
     engine = (llm_engine
               if llm_engine is not None else AsyncLLMEngine.from_engine_args(
                   engine_args, usage_context=UsageContext.API_SERVER))
-
+    app.state.engine_client = engine
     return app