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

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

content:
  application/json:
    schema:
      properties:
        id:
          oneOf:
            - type: integer
            - type: string
      required:
        - id
      type: object

to:

content:
  application/json:
    schema:
      properties:
        id:
          type: integer
      required:
        - id
      type: object

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:
                    oneOf:
                      - type: integer
                      - type: string
                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:
                  type: integer
              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 breaking base.yml revision.yml
3. Observe the following output:

==========================================================================
==                            API CHANGE LOG                            ==
==========================================================================
                               User Service
--------------------------------------------------------------------------
--                            What's Changed                            --
--------------------------------------------------------------------------
- POST   /users
  Return Type:
    - Changed 201 Created
      Media types:
        - Changed application/json
          Schema: Broken compatibility
          Changed property type: id (object -> integer)
--------------------------------------------------------------------------
--                                Result                                --
--------------------------------------------------------------------------
                 API changes broke backward compatibility
--------------------------------------------------------------------------

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

Bắt đầu với các tệp base.yml và revision.yml được cung cấp, rồi chạy openapi-diff breaking base.yml revision.yml để tái hiện kết quả tương thích đã báo cáo. Lần theo đường dẫn so sánh schema cho thuộc tính response id, sau đó bổ sung coverage cho thấy việc thu hẹp oneOf thành type: integer được chấp nhận và không có kết quả breaking change nào được báo cáo.