[BUG][Java][serializableModel] Generated inner-`enum` lack `implements Serializable`

[Chrimle]

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• Have you validated the input using an OpenAPI validator?
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Description

Inner enums lack implements Serializable, while outer/standalone does not.
Discovered here: Chrimle/openapi-to-java-records-mustache-templates#578
Same root cause as #23083

openapi-generator version

7.20.0

OpenAPI declaration file content or url

components:
  schemas:
    ExampleWithInnerEnums:
      type: object
      description: Example with inner enum class
      properties:
        exampleInner:
          type: string
          description: Example of an inner enum class
          enum:
            - ENUM1
            - ENUM2
            - ENUM3

Generates:

  @JsonAdapter(ExampleInnerEnum.Adapter.class)
  public enum ExampleInnerEnum {

But should be:

  @JsonAdapter(ExampleInnerEnum.Adapter.class)
  public enum ExampleInnerEnum implements Serializable {

Generation Details

Library: java
serializableModel: true

Steps to reproduce

Related issues/PRs

• [BUG][Java][x-implements] Generated inner-enum lack implements #23083

Suggest a fix