fix: [OpenAPI] Fix Enum Discriminator

[MatKuhr]

Context

Fixes a compilation issue in case enum is used together with a discriminator.

Background: This is a frequent use case, as people want to explicitly limit the allowed values for the discriminator. In OpenAPI 3.0, this is done by declaring an enum with just one value.

Definition of Done

• Functionality scope stated & covered
• Tests cover the scope above
• Error handling created / updated & covered by the tests above
• Documentation updated
• Release notes updated

[newtork]

It's a breaking change, but since it's behind a feature toggle, presumably that's acceptable.