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

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

Inner enums lack implements Serializable, while outer/standalone does not.
Discovered here: https://github.com/Chrimle/openapi-to-java-records-mustache-templates/issues/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

• #23083

Suggest a fix

Start by reproducing Java generation with the supplied schema and the serializableModel option, then inspect where outer and inner enum declarations are produced. Confirm that the generated inner enum implements Serializable as expected, while preserving the existing adapter and model output.