Annotate nullable values as such in the OpenAPI schema (#2316)
* Annotate nullable values as such in the OpenAPI schema
Prior to this change, we allowed the return of `None` values from
models, serializing them as `null`, but the schema didn't specify them
as nullable, which would cause model output to fail validation.
This only supports Pydantic V2, because, from what I can tell, Pydantic
V1 doesn't give us any clues that a field allows `None` values.
* Support nullable: true in pydantic v1
* Support output types nullable in pydantic v1
* Fix lint
* Remove input_seed
* Simplify nullable fields
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Co-authored-by: Will Sackfield <sackfield@replicate.com>
