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/v1/test_serial_utils.py

@@ -2,7 +2,6 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 from collections import UserDict
 from dataclasses import dataclass
-from typing import Optional
 
 import msgspec
 import numpy as np
@@ -100,7 +99,7 @@ def test_encode_decode(monkeypatch: pytest.MonkeyPatch):
 
 
 class MyRequest(msgspec.Struct):
-    mm: Optional[list[MultiModalKwargsItems]]
+    mm: list[MultiModalKwargsItems] | None
 
 
 def test_multimodal_kwargs():