[php-nextgen] Fix for enum allowed values, fixes #23813

[coffeemakr]

I added

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Summary by cubic

Fixes incorrect enum allowed values in php-nextgen models and restores missing discriminator constants. Prevents mixing discriminator values into unrelated enums. Fixes #23813.

• Bug Fixes
  • Update model_generic.mustache to handle enums separately for discriminator vs non-discriminator fields and preserve discriminator constants.
  • Add tests for oneOf discriminator enum generation and constant preservation; include repro spec src/test/resources/bugs/issue_23813.yaml.

^{Written for commit 14e75a6. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[wing328]

thanks for the PR

cc @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)

cc @JulianVennen (top contributor to php-nextgen)

[JulianVennen]

I think the entire discriminator stuff in nextgen has to be overhauled at some point as it doesn't really work the way it should.

The object serializer always returns a PetAnimal, because:

1. The mapping key is not implemented. Discriminator values have to be the literal class name of the model
2. The generated classes are not children of the PetAnimal class.

In other generators (e.g. typescript) those work correctly.

That being said, this PR does seem to be an improvement for now until someone has time and motivation to implement the discriminator stuff properly.