[BUG] [scala-sttp] [scala-akka] OpenAPI generator does not generate additional properties of type Object

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When I use the scala-sttp / scala-akka generators, the case classes generated for JSON models in OpenAPI spec do not include the additional properties of type Object. Details follow.

Here is the OpenAPI snippet showing the Element JSON model. Note the additionalProperties at the end of type Object.

Element:
    type: object
    properties:
      '@type':
        type: string
      '@id':
        type: string
        format: uuid
      identifier:
        type: string
        format: uuid
    additionalProperties:
      type: object

Here is the case class for Element generated when using scala-sttp and scala-akka generators. As you can see, there is no generic property in the case class that is of type Map to hold the additional properties in Element.

import java.util.UUID
import org.openapitools.client.core.ApiModel

case class Element (
  `type`: Option[String] = None,
  id: Option[UUID] = None,
  identifier: Option[UUID] = None
) extends ApiModel

On the contrary, the Java generator handles this correctly. Element class inherits HashMap.

public class Element extends HashMap<String, Object> {
  public static final String SERIALIZED_NAME_AT_TYPE = "@type";
  @SerializedName(SERIALIZED_NAME_AT_TYPE)
  private String atType;

  public static final String SERIALIZED_NAME_AT_ID = "@id";
  @SerializedName(SERIALIZED_NAME_AT_ID)
  private UUID atId;

  public static final String SERIALIZED_NAME_IDENTIFIER = "identifier";
  @SerializedName(SERIALIZED_NAME_IDENTIFIER)
  private UUID identifier;
}

openapi-generator version

OpenAPI 4.3.1. It is not a regression. This has never worked for us.

OpenAPI declaration file content or url

See above in the Description.

Generation Details

scala-sttp command used
java -jar openapi-generator-cli-4.3.1.jar generate --api-package com.org.api --model-package com.org.model -i my_openapi.yaml -g scala-akka -o ./output/scala-sttp

scala-akka command used
java -jar openapi-generator-cli-4.3.1.jar generate --api-package com.org.api --model-package com.org.model -i my_openapi.yaml -g scala-akka -o ./output/scala-akka

java command used
java -jar openapi-generator-cli-4.3.1.jar generate --api-package com.org.api --model-package com.org.model -i my_openapi.yaml -g scala-akka -o ./output/scala-java

Steps to reproduce

Provided in the Description above.

Related issues/PRs

None that I have seen.

Suggest a fix

No suggestions.

Start by reproducing the issue with the provided OpenAPI Element schema and the scala-sttp and scala-akka generator commands. Inspect those generator entry points and compare their model output with the Java generator's handling of additionalProperties. Done means the generated Scala case classes preserve arbitrary Object-valued additional properties and the behavior is covered by the relevant generator tests.