$ref changes are dropped when a document has any circular reference #662

If a document contains any circular reference, CheckSchemaProxyForCircularRefs returns true for every schema proxy, including ones that are not part of the loop. CompareSchemas then returns nil in its $ref branches, so changes to those schemas are not reported at all.

# old.yaml
openapi: 3.0.3
info: {title: t, version: '1'}
paths: {}
components:
  schemas:
    Owner:
      type: object
      properties:
        pet:
          $ref: '#/components/schemas/Pet'
    Pet:
      type: object
      properties:
        name: {type: string}
    A:
      type: object
      properties:
        b: {$ref: '#/components/schemas/B'}
    B:
      type: object
      properties:
        a: {$ref: '#/components/schemas/A'}

new.yaml is the same, except Owner.pet is inlined and requires name:

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

openapi-changes report old.yaml new.yaml reports no changes. Remove A and B from both files and it reports the $ref change.

The cause is that the journey check (source) keeps one dup map across all circular references and returns true on any repeated entry, without comparing it to s. A journey always repeats its loop point (B -> A -> B), so the first circular reference makes the function return true.

A fix needs care. The dup check is currently also what stops QuickHash from recursing through cross-file cycles. A relative $ref such as b.yaml#/B doesn't match the journey's Definition or FullDefinition, so a narrower check that compares s.GetReference() hangs on those documents. A journey also includes the path leading into the loop, not just the loop itself. So the check probably needs to compare the proxy's resolved full definition against Journey[LoopIndex:]. Does that direction make sense to you? Happy to work on a PR once we agree on the approach.

Related to #663