[BUG] [Kotlin] when using discriminator pattern in Kotlin generator the data classes don't implement the Interface

[luismospinam]

Description

Related to: #13484
In Kotlin, when using discriminator property, the data/concrete classes don't implement the generated interface.

With the following openapi.yaml spec:

openapi: "3.0.3"
info:
  title: MS Pet API
  version: 1.0.0
  description: De API for testing the discriminator in openapi
paths:
  /api/projects:
    get:
      summary: get a pet
      operationId: getPets
      tags:
        - pets
      responses:
        200:
          description: Succes respons
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/PetResponse'

components:
  schemas:
    PetResponse:
      oneOf:
        - $ref: '#/components/schemas/Cat'
        - $ref: '#/components/schemas/Dog'
      discriminator:
        propertyName: petType
    Cat:
      type: object
      # all other properties specific to a `Cat`
      required:
        - petType
      properties:
        catName:
          type: string
        petType:
          type: string
    Dog:
      type: object
      # all other properties specific to a `Dog`
      required:
        - petType
      properties:
        bark:
          type: string
        petType:
          type: string

Generates the following classes and interface in Kotlin:

interface PetResponse {
    @Json(name = "petType")
    val petType: kotlin.String

    @Json(name = "catName")
    val catName: kotlin.String?

    @Json(name = "bark")
    val bark: kotlin.String?
}

data class Cat (
    @Json(name = "petType")
    val petType: kotlin.String,

    @Json(name = "catName")
    val catName: kotlin.String? = null
)

data class Dog (
    @Json(name = "petType")
    val petType: kotlin.String,

    @Json(name = "bark")
    val bark: kotlin.String? = null
)

As seen, the data classes (Dog and Cat) don't implement the interface PetResponse, even when using the following property config:

additionalProperties.set(
        mapOf(
            "useOneOfInterfaces" to "true",
        ),
    )

Here's a simple project to demonstrate the issue: https://github.com/luismospinam/Openapi-generator-kotlin-discriminator
run ./gradlew openApiGenerate