[BUG][GO] JSON marshalling incorrect when setting anyOf properties as required

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?
  Expecting marshalling to produce

  {"obj":"A"} 
  

  But it produces

  {"obj":{"Int32":null,"String":"A"}}
  

• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When an anyOf model is a required property, JSON marshalling is incorrect.

openapi-generator version

docker run --rm openapitools/openapi-generator-cli:latest version
7.10.0-SNAPSHOT

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  version: 1.0.0
  title: FooBar Service
paths: {}
components:
  schemas:
    Bad:
      type: object
      description: bad
      required:
        - obj
      properties:
        obj:
          $ref: '#/components/schemas/Obj'
    Obj:
      anyOf:
        - type: string
        - type: integer
      example: a
      description: obj

Generation Details

I'm running

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:latest generate \
        --input-spec /local/go-json-marshal.yaml \
        --generator-name go \
        --additional-properties "withGoMod=false" \
        --package-name gen \
        --output /local/gen

Steps to reproduce

Run this gist

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/pull/19962
• https://github.com/OpenAPITools/openapi-generator/issues/14028

Suggest a fix

• https://github.com/OpenAPITools/openapi-generator/pull/19962

Generate the Go client from the YAML declaration using the Docker command in the report, then run the linked gist to reproduce the marshalling output. Compare the required anyOf property with the expected {"obj":"A"} result and the nested output shown. Done means the generated Go JSON marshalling matches the expected representation for this specification.