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.

Inizia con i file base.yml e revision.yml forniti ed esegui openapi-diff breaking base.yml revision.yml per riprodurre il risultato di compatibilità segnalato. Segui il percorso di confronto dello schema per la proprietà di risposta id, quindi aggiungi una copertura che mostri che restringere oneOf a type: integer è accettato e che non viene segnalato alcun risultato di modifica incompatibile.