[Bugfix] Fix pydantic.errors.PydanticUserError (#17962)

Signed-off-by: wangli <wangli858794774@gmail.com>

vllm/entrypoints/openai/serving_engine.py

 # SPDX-License-Identifier: Apache-2.0
 
 import json
+import sys
 import time
 from collections.abc import (AsyncGenerator, Iterable, Iterator, Mapping,
                              Sequence)
 from concurrent.futures.thread import ThreadPoolExecutor
 from http import HTTPStatus
 from typing import (Annotated, Any, Callable, ClassVar, Generic, Optional,
-                    TypedDict, TypeVar, Union)
+                    TypeVar, Union)
 
 from fastapi import Request
 from pydantic import BaseModel, ConfigDict, Field
 from starlette.datastructures import Headers
 
+if sys.version_info >= (3, 12):
+    from typing import TypedDict
+else:
+    from typing_extensions import TypedDict
+
 import vllm.envs as envs
 from vllm.config import ModelConfig
 from vllm.engine.protocol import EngineClient