Add explicit OpenAPI schemas for Response endpoints #12705

This is a followup to PR https://github.com/IQSS/dataverse/pull/12439. SmallRye OpenAPI plugin generator has been updated to 4.3.5, which generates empty schema types.

This PR documents opaque Response endpoints with explicit JSON object and string response schemas so SmallRye emits valid OpenAPI types. This removes 86 oas-missing-type findings and raises Vacuum quality from 89 to 98.

No Java return signatures or runtime behavior changed.

Which issue(s) this PR closes:

Closes https://github.com/IQSS/dataverse/issues/12701

Suggestions on how to test this:

Use same Vacuum settings as in https://github.com/IQSS/dataverse/pull/12439

Should we maybe add some form of vacuum test to the dataverse test suite, so that we are always aware of OpenAPI regressions?