Breaking changes are missed when a schema $ref is replaced or retargeted #663

When a schema $ref is replaced by an inline schema, an inline schema by a $ref, or a $ref is pointed at another schema, CompareSchemas records a non-breaking $ref change and returns. The new schema is never compared, so breaking changes in it are missed.

# old.yaml
openapi: 3.1.1
info: {title: Pets, version: '1'}
paths:
  /pets:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Pet'
      responses:
        '200': {description: The pet}
components:
  schemas:
    Pet:
      type: object
      properties:
        name: {type: string}

new.yaml is the same, except the request body schema is inlined and requires name:

            schema:
              type: object
              properties:
                name: {type: string}
              required: [name]

openapi-changes report old.yaml new.yaml (0.2.13) reports 1 change, 0 breaking. The same required change between two inline schemas is reported as breaking. TestCompareSchemas_PropertyRefChange_Fail shows the same thing: a $ref to a type: int schema becomes inline type: string, and only the non-breaking $ref change is reported.

Removing the early return sc isn't enough: it was added to stop over-cycling (TestCheckExplodedFileCheck asserts the single $ref change), and falling through re-compares shared subtrees once per path, which grows exponentially when a tree of schemas is renamed. It also bypasses the object→composition handling from #650, so $ref: Pet → anyOf: [$ref Pet, null] reports false breaking changes. A fix probably needs to remember which schema pairs were already compared. How would you like this approached? Happy to help with a PR.

Related: #662