Changing request field type from `string` to `oneOf: string, number` shouldn't break

base.yml と revision.yml を使い、openapi-diff base.yml revision.yml でレポートを再現してから、name プロパティに対するリクエストボディスキーマの互換性処理を追跡します。integer から integer と string を含む oneOf に変更しても、breaking change として表示されなくなれば完了です。

Describe the bug
In the request body, if you change the following field:

requestBody:
  content:
    application/json:
      schema:
        type: object
        properties:
          name:
            type: integer
        required:
          - name
  required: true

to:

requestBody:
  content:
    application/json:
      schema:
        type: object
        properties:
          name:
            oneOf:
              - type: integer
              - type: string
        required:
          - name
  required: true

Then openapi-diff reports this as a breaking change.

To Reproduce

openapi: 3.0.1
info:
  title: User Service
  version: 1.0.0
paths:
  /users:
    post:
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                name:
                  type: integer
              required:
                - name
        required: true
      responses:
        201:
          description: Created
          content:
            application/json:
              schema:
                properties:
                  id:
                    type: integer
                required:
                  - id
                type: object

openapi: 3.0.1
info:
  title: User Service
  version: 1.0.0
paths:
  /users:
    post:
      requestBody:
        content:
          application/json:
            schema:
              type: object
              properties:
                name:
                  oneOf:
                    - type: integer
                    - type: string
              required:
                - name
        required: true
      responses:
        201:
          description: Created
          content:
            application/json:
              schema:
                properties:
                  id:
                    type: integer
                required:
                  - id
                type: object

1. Download the two files base.yml and revision.yml
2. Run openapi-diff base.yml revision.yml
3. Observe the following output:

==========================================================================
==                            API CHANGE LOG                            ==
==========================================================================
                               User Service
--------------------------------------------------------------------------
--                            What's Changed                            --
--------------------------------------------------------------------------
- POST   /users
  Request:
        - Changed application/json
          Schema: Broken compatibility
          Changed property type: name (integer -> object)
--------------------------------------------------------------------------
--                                Result                                --
--------------------------------------------------------------------------
                 API changes broke backward compatibility
--------------------------------------------------------------------------

Expected behavior
openapi-diff shouldn't mark this as a breaking change. Actually, the request body should be considered as a contravariant contract: widening a field type isn't a breaking change.