fix(marshmallow): serialize OneOf and Equal validator choices to OpenAPI enum (closes #679)

[saifmodan2006]

Summary

Fixes #679.

When using marshmallow fields with \�alidate.OneOf\ or \�alidate.Equal\ constraints containing deserialized objects (such as \Enum\ instances, \datetime.date, \datetime.datetime, \uuid.UUID, \datetime.timedelta, etc.), \ield2choices\ was previously inserting the raw Python objects directly into the OpenAPI \enum\ list. This resulted in non-serializable objects and broke JSON schema generation.

Changes

1. Added _serialize_choice(field, choice)\ helper in \FieldConverterMixin:
   • Serializes choices using \ arget_field._serialize(choice, None, None)\ (using \ield.inner\ when \ield\ is a \ields.List).
   • Ensures the serialized choice is hashable and JSON-compatible, falling back safely to \choice\ if serialization is not supported or raises an exception.
2. Updated \ield2choices:
   • Serializes validator choices for both \�alidate.OneOf\ (\�alidator.choices) and \�alidate.Equal\ (\�alidator.comparable).
3. Updated \enum2properties:
   • Preserves restricted \enum\ lists populated by \ield2choices\ from field validators rather than unconditionally overwriting them with all enum members.
4. Added comprehensive regression tests in \ ests/test_ext_marshmallow_field.py\ covering choice serialization for dates, datetimes, UUIDs, Equal validators, labels, and Enum fields with OneOf restrictions.

[sloria]

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