[BUG][Java][okhttp-gson] oneOf support is failing by not setting and validate discriminator type

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

1. Okhttp-gson client fails by not sending the discriminator type during communication with a backend.

• Expected result:
  Client sends discriminator type correctly, and server can deserialize correctly.

• Actual result:

Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error:
     Could not resolve subtype of [simple type, class server.api.PhysicalMountingApiModel]: missing type id property 'object_type' (for POJO property 'mounting')]

2. Sending List with multiple oneOf from server (server sends the discriminator "object_type" correctly) - the client fails during property validation phase.

• Expected result:
  Client can deserialize server payload into correct objects based on discriminator.

• Actual result:

     2 class(es) match the result, expected 1.
     Detailed failure message for oneOf schemas: [Deserialization for FloorMounting failed with`The required field `orientation` is not found in the JSON string: {"object_type":"LocationMounting","mountable_type":"FIXED"}`.].
     JSON: {"object_type":"LocationMounting","mountable_type":"FIXED"}

useOneOfDiscriminatorLookup has been enabled for okhttp-gson client.

openapi-generator version

6.2.1

OpenAPI declaration file content

openapi: 3.0.3
info:
  contact:
    email: email@email.com
    name: name
    url: https://whatever.com
  description: Internal API
  title: Internal API
  version: 1.0.0
  x-audience: business-unit-internal
  x-api-id: 3addb128-c3fb-4bfe-a5d9-c84b3bd58ca2
servers:
- description: Local environment
  url: http://localhost:9090/api
paths:
  /authenticated/organizations/aspects/physical-aspects/:
    get:
      operationId: listAll
      responses:
        "200":
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/PhysicalAspectsResponse'
          description: List of the physicals
      summary: List all physicals
      tags:
      - Physical
      x-accepts: application/json
    put:
      operationId: update
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/UpdatePhysicalRequest'
      responses:
        "200":
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/PhysicalAspectResponse'
          description: Physical aspect updated successfully
      summary: Update Physical aspect
      tags:
      - Physical
      x-content-type: application/json
      x-accepts: application/json
components:
  schemas:
    PhysicalAspectsResponse:
      example:
        physicals:
        - identifiable_id: 046b6c7f-0b8a-43b9-b35d-6489e6daee91
          mounting: null
        - identifiable_id: 046b6c7f-0b8a-43b9-b35d-6489e6daee91
          mounting: null
      properties:
        physicals:
          items:
            $ref: '#/components/schemas/PhysicalAspect'
          type: array
      required:
      - physicals
      type: object
    UpdatePhysicalRequest:
      example:
        physical:
          identifiable_id: 046b6c7f-0b8a-43b9-b35d-6489e6daee91
          mounting: null
      properties:
        physical:
          $ref: '#/components/schemas/PhysicalAspect'
      required:
      - physical
      type: object
    PhysicalAspectResponse:
      example:
        physical:
          identifiable_id: 046b6c7f-0b8a-43b9-b35d-6489e6daee91
          mounting: null
      properties:
        physical:
          $ref: '#/components/schemas/PhysicalAspect'
      required:
      - physical
      type: object
    PhysicalAspect:
      example:
        identifiable_id: 046b6c7f-0b8a-43b9-b35d-6489e6daee91
        mounting: null
      properties:
        identifiable_id:
          format: uuid
          type: string
        mounting:
          $ref: '#/components/schemas/PhysicalMounting'
      required:
      - definition
      - identifiable_id
      - mountable
      - mounting
      type: object
    PhysicalMounting:
      discriminator:
        propertyName: object_type
      oneOf:
      - $ref: '#/components/schemas/NotMountedMounting'
      - $ref: '#/components/schemas/FloorMounting'
      - $ref: '#/components/schemas/LocationMounting'
      type: object
    NotMountedMounting:
      properties:
        object_type:
          type: string
        mountable_type:
          $ref: '#/components/schemas/MountableType'
      required:
      - object_type
      type: object
    FloorMounting:
      properties:
        object_type:
          type: string
        mountable_type:
          $ref: '#/components/schemas/MountableType'
        orientation:
          format: float
          maximum: 360
          minimum: -360
          type: number
      required:
      - object_type
      - orientation
      type: object
    LocationMounting:
      properties:
        object_type:
          type: string
        mountable_type:
          $ref: '#/components/schemas/MountableType'
      required:
      - object_type
      type: object
    MountableType:
      enum:
      - FIXED
      - LOCATION
      type: string

Steps to reproduce

I've included a reproducible test project based on a Maven project, Spring Boot 2.7.x and Java 17 using okhttp, jersey2 and spring-cloud libraries for comparison.
okhttp-gson-jersey2-springcloud-testcase.zip

Testcases are placed in Jersey2Test.java, OkHttpTest.java and SpringCloudTest.java

Download the testcase and mvn clean install. Testcases can be found in #DemoApplicationTests

cc @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

Download the linked Maven testcase and run mvn clean install; begin with OkHttpTest.java and the cases in DemoApplicationTests, comparing them with Jersey2Test.java and SpringCloudTest.java. Trace the okhttp-gson handling of the oneOf discriminator. Done means requests send object_type and responses deserialize each payload into the correct subtype without validation ambiguity.