[Frontend] Delegate preprocessing to OpenAIServingRender (#36483)

Signed-off-by: Sage Ahrac <sagiahrak@gmail.com>

tests/v1/engine/test_async_llm.py

@@ -508,11 +508,25 @@ async def test_header_dp_rank_argument():
             base_model_paths=BASE_MODEL_PATHS,
         )
 
+        # Create render serving instance (required by OpenAIServingChat)
+        from vllm.entrypoints.serve.render.serving import OpenAIServingRender
+
+        serving_render = OpenAIServingRender(
+            model_config=engine.model_config,
+            renderer=engine.renderer,
+            io_processor=engine.io_processor,
+            model_registry=models.registry,
+            request_logger=None,
+            chat_template=None,
+            chat_template_content_format="auto",
+        )
+
         # Create serving chat instance
         serving_chat = OpenAIServingChat(
             engine_client=engine,
             models=models,
             response_role="assistant",
+            openai_serving_render=serving_render,
             chat_template=None,
             chat_template_content_format="auto",
             request_logger=None,