Add type safe returns

[louislepper]

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Summary by cubic

Adds opt-in sealed response interfaces for Kotlin Spring to make controller return types type-safe. Each operation gets an Response interface, implemented by its response models, and API methods return that interface when enabled.

• New Features

  • New option: useSealedResponseInterfaces (default: false).
  • Generates SealedResponseInterfaces.kt in the model package (e.g., CreateUserResponse).
  • Models used in responses automatically implement the right interfaces.
  • API methods return the sealed interface and import it (works with ResponseEntity).
  • Updated templates: apiInterface.mustache, dataClass.mustache, and sealedResponseInterfaces.mustache.
  • Tests added for enabled/disabled behavior with a dedicated test spec.
  • New petstore sample and config (bin/configs/kotlin-spring-sealed-interfaces.yaml) to demonstrate usage.
  • Docs updated to document the option.

• Migration

  • Opt-in by setting useSealedResponseInterfaces=true and regenerate.
  • Update controller implementations to return the generated sealed interface type.
  • No changes required if the option is not enabled.

^{Written for commit c674e09. Summary will update on new commits.}