Update Optional[x] -> x | None and Union[x, y] to x | y (#26633)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

tests/entrypoints/openai/test_completion_with_function_calling.py

@@ -2,7 +2,6 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
 import datetime
-from typing import Union
 
 import openai  # use the official client for correctness check
 import pytest
@@ -166,7 +165,7 @@ async def test_function_tool_use(
     client: openai.AsyncOpenAI,
     model_name: str,
     stream: bool,
-    tool_choice: Union[str, dict],
+    tool_choice: str | dict,
     enable_thinking: bool,
 ):
     if not stream: