[Frontend] Delegate preprocessing to OpenAIServingRender (#36483)

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

tests/entrypoints/openai/test_chat_error.py

@@ -13,6 +13,7 @@ from vllm.entrypoints.openai.chat_completion.serving import OpenAIServingChat
 from vllm.entrypoints.openai.engine.protocol import GenerationError
 from vllm.entrypoints.openai.models.protocol import BaseModelPath
 from vllm.entrypoints.openai.models.serving import OpenAIServingModels
+from vllm.entrypoints.serve.render.serving import OpenAIServingRender
 from vllm.outputs import CompletionOutput, RequestOutput
 from vllm.renderers.hf import HfRenderer
 from vllm.tokenizers.registry import tokenizer_args_from_config
@@ -84,10 +85,20 @@ def _build_serving_chat(engine: AsyncLLM) -> OpenAIServingChat:
         engine_client=engine,
         base_model_paths=BASE_MODEL_PATHS,
     )
+    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",
+    )
     serving_chat = OpenAIServingChat(
         engine,
         models,
         response_role="assistant",
+        openai_serving_render=serving_render,
         request_logger=None,
         chat_template=None,
         chat_template_content_format="auto",
@@ -100,7 +111,9 @@ def _build_serving_chat(engine: AsyncLLM) -> OpenAIServingChat:
             [{"prompt_token_ids": [1, 2, 3]}],
         )
 
-    serving_chat._preprocess_chat = AsyncMock(side_effect=_fake_preprocess_chat)
+    serving_chat.openai_serving_render._preprocess_chat = AsyncMock(
+        side_effect=_fake_preprocess_chat
+    )
     return serving_chat