Update _models.py to handle Python 3.14

Author: Natim

src/openai/_models.py

@@ -2,7 +2,7 @@
 
 import os
 import inspect
-from typing import TYPE_CHECKING, Any, Type, Tuple, Union, Generic, TypeVar, Callable, Optional, cast
+from typing import TYPE_CHECKING, Any, Annotated, Type, Tuple, Union, Generic, TypeVar, Callable, Optional, cast
 from datetime import date, datetime
 from typing_extensions import (
     List,
@@ -694,7 +694,7 @@ def _build_discriminated_union_meta(*, union: type, meta_annotations: tuple[Any,
         discriminator_field=discriminator_field_name,
         discriminator_alias=discriminator_alias,
     )
-    cast(CachedDiscriminatorType, union).__discriminator__ = details
+    cast(CachedDiscriminatorType, Annotated[union, details])
     return details