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.

从提供的 base.yml 和 revision.yml 文件开始，运行 openapi-diff breaking base.yml revision.yml 以复现报告的兼容性结果。跟踪响应属性 id 的模式比较路径，然后添加覆盖测试，表明将 oneOf 收窄为 type: integer 会被接受，并且不会报告破坏性变更结果。