ModelResponse.choices was typed as List[Union[Choices, StreamingChoices]] which
caused Pydantic serialization warnings and false linting errors. Now that
ModelResponseStream exists for streaming, narrow ModelResponse.choices to
List[Choices] and migrate all ModelResponse(stream=True) call sites to use
ModelResponseStream() instead.
Pydantic v2's Union serializer for `List[Union[Choices, StreamingChoices]]`
tries both branches when serializing, emitting spurious
`PydanticSerializationUnexpectedValue` warnings (field count mismatch on
`Message` and type mismatch `Expected StreamingChoices but got Choices`).
Add a `WrapSerializer` on the `choices` field that serializes each item
individually via its own `model_dump()`, bypassing the Union dispatch
entirely while correctly propagating `exclude_none`, `exclude_unset`,
and `exclude_defaults` from the parent serialization context.
Ishaan Jaffer