[BUG] [Core] anyOf/oneOf marks union of required fields as required, should be intersection

[winrid]

The merged composed model from anyOf / oneOf adds every member's required fields to required. A field is only guaranteed present if every member requires it, so the merged set should be the intersection. Affects every generator that uses CodegenModel.required (Swift, Kotlin, Java POJOs, Dart, etc.).

response:
  anyOf:
    - $ref: '#/components/schemas/VoteResponse'   # required: [status, voteId]
    - $ref: '#/components/schemas/APIError'       # required: [status, reason, code]

Currently the generator marks status, voteId, reason, code all required; only status is. Working on a PR.