[kotlin-server] Enable oneOf sealed interface generation

[mcuk-globalid]

Summary

The kotlin-server generator produces typealias Foo = Any for oneOf schemas with discriminators instead of a sealed interface. This causes compile errors when subtypes (generated via allOf) try to extend the parent type.

Reproducer: any OpenAPI 3.1.x spec with oneOf + discriminator + allOf subtype pattern (e.g. src/test/resources/3_1/polymorphism-and-discriminator.yaml) generates:

typealias Pet = Any

instead of:

sealed interface Pet { ... }

Subtypes then fail to compile because Any has a constructor that cannot be delegated to.

Root cause: KotlinServerCodegen declares SchemaSupportFeature.oneOf and SchemaSupportFeature.Polymorphism but does not set useOneOfInterfaces = true, so the codegen pipeline never produces the x-is-one-of-interface vendor extension. The model.mustache template also lacks the routing to a oneof_interface template.

Fix

Mirrors #23574 (kotlin-spring):

1. Set useOneOfInterfaces = true in KotlinServerCodegen constructor
2. Add oneof_interface.mustache template for kotlin-server (sealed interface with Jackson @JsonTypeInfo / @JsonSubTypes annotations)
3. Update model.mustache to route x-is-one-of-interface models to the new template

Related issues

• [BUG][KOTLIN] oneOf does not generate type hierarchy correctly #22013 — [BUG][KOTLIN] oneOf does not generate type hierarchy correctly
• [BUG] [Kotlin] when using discriminator pattern in Kotlin generator the data classes don't implement the Interface #19928 — [BUG][Kotlin] discriminator pattern data classes don't implement the Interface
• [BUG] Discriminator classes don't implement parent interface (in Kotlin generator) #20260 — [BUG] Discriminator classes don't implement parent interface

🤖 Generated with Claude Code

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Summary by cubic

Generate sealed interfaces for oneOf schemas in kotlin-server instead of typealias Any. Restores correct polymorphism and fixes compile errors for allOf subtypes.

• Bug Fixes
  • Enable useOneOfInterfaces = true in KotlinServerCodegen.
  • Add oneof_interface.mustache to render a sealed interface with Jackson @JsonTypeInfo/@JsonSubTypes.
  • Route x-is-one-of-interface in both model.mustache and libraries/jaxrs-spec/model.mustache.
  • Emit parent constructor args only when needed (data_class.mustache); skip when generating interfaces.
  • Update tests to expect sealed interfaces and use TestUtils.parseSpec; regenerate affected samples.

^{Written for commit f2d564f. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 4 files

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