[core] Resolve oneOf discriminator type when property is inherited by alternatives

[Ignacio-Vidal]

Problem

When a oneOf/anyOf interface declares the discriminator on itself but does not define the discriminator property locally, the discriminator property type was resolved as String instead of the actual type.

This happens when each alternative inherits the discriminator property from a shared base via allOf, while the discriminator keyword lives on the oneOf interface schema rather than on the base.

Concrete repro (jaxrs-spec, useOneOfInterfaces):

• PetRequestV2 declares properties.petType: $ref PetType inline on the schema → generated PetType getPetType(); ✅
• PetRequestV3 has no properties on the schema; petType is defined in a shared base referenced by each alternative's allOf → generated String getPetType(); ❌

Upstream PR OpenAPITools#24158 fixed the related case where the discriminator lives on the shared base and is inherited by the alternatives, but not this variant where the discriminator sits on the oneOf interface itself.

Fix

DiscriminatorUtils.recursiveGetDiscriminator now descends into the schema's oneOf/anyOf/allOf alternatives (cycle-guarded) to discover the discriminator property schema when the interface schema does not define it locally. The generated interface getter then uses the concrete type (e.g. an enum) rather than falling back to String.

The change is a strictly additive fallback: it only fires when the previous code returned null for the discriminator schema (which is exactly the case that fell back to String).

Tests

• New testOneOfDiscriminatorOnInterfacePropertyInAlternatives in DefaultCodegenTest asserting the resolved type is the enum (PetType) and getIsEnum() is true.
• New schemas (PetRequestV3, PetV3Base, CatRequestV3, DogRequestV3) in oneOfDiscriminator.yaml modelling the discriminator-on-interface + property-in-alternatives pattern.

Verification

• DefaultCodegenTest (171), JavaJAXRSSpecServerCodegenTest (148), plus JavaClientCodegenTest, SpringCodegenTest, JavaHelidonCommonCodegenTest, JavaMicronautClientCodegenTest — all pass, no regressions.
• Regenerated a real-world contract end-to-end: PetRequestV3 now emits PetType getPetType();; PetRequestV2 unchanged.

Note: samples were not regenerated in this PR — worth confirming via CI whether any committed sample matches this pattern.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Lw5PwrLknPHfsnkwTu2N1B