Support enums in `transform_schema`

[s-banach]

For example, pydantic models with Literal fields get translated into str type fields with the enum values merely hinted at in the description.

The backend supports constrained generation using the enums, so the enum type should be preserved.

Another user has proposed a fix already:
#1178
(Not to endorse the particular fix, just to say this is an issue other people have noticed.)