[Scala] Add enums to scala-http4s-server

[KatyaValik]

There is an example of correct openAPI specification:

info:
  title: TestApi
  version: latest

components:
  schemas:
    EnumType:
      type: string
      enum:
        - Fix
        - Max
paths:
  /test/method:
    post:
      responses:
        '200':
          description: "success answer"
          content:
            application/json:
              schema:
                type: object
                required:
                  - constraint
                properties:
                  constraint:
                    $ref: '#/components/schemas/EnumType'

For given OpenAPI specification generated not valid type for enum EnumType - empty case class:

case class EnumType()

I added enum handling for scala-http4s-server.
Trying to fix #9987 for scala-http4s-server.

[wing328]

thanks for the PR

cc
@clasnake (2017/07), @shijinkui (2018/01), @ramzimaalej (2018/03), @chameleon82 (2020/03), @Bouillie (2020/04) @Fish86 (2023/06)

[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/15276044149/job/42973797933?pr=21320

please follow step 3 to update the samples so that the CI can verify the change