[kotlin] Set required fields to optional when multiple anyOf are used

FO
2d ago
+474-94

Currently, for Kotlin generators, when an object has multiple anyOf with differing required fields, it generates an empty data class which causes a Kotlin compiler error.
This change makes the set of required fields between each anyOf branch optional.
This fixes #18626

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Summary by cubic

Fixes #18626, where Kotlin models for anyOf schemas with differing required fields could generate empty data classes and fail compilation. Properties required in only some branches are now optional and nullable, while properties required across all branches stay required; constructor properties preserve schema order. This changes affected model APIs by adding nullable types and default values to branch-specific properties.

  • Requiredness is resolved through refs, allOf, and nested anyOf/oneOf branches, and discriminator-sealed parents use the adjusted constructor.
  • Applies to kotlin-server, ktor2, kotlin-spring, and kotlin-vertx-server templates.