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

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.

Tái hiện báo cáo với base.yml và revision.yml bằng openapi-diff base.yml revision.yml, sau đó lần theo cách xử lý khả năng tương thích của schema request body đối với thuộc tính name. Được xem là hoàn tất khi việc thay đổi integer thành oneOf với integer và string không còn xuất hiện dưới dạng breaking change.