Fix allOf with a single $ref

[wing328]

• allOf with a single $ref
• add tests

to fix #16047

FYI @OpenAPITools/generator-core-team

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[y-chan]

LGTM!

[rob-spoor]

Is there an ETA on a release with this fix included? I need to migrate away from 5.1.1 because I need to add Jakarta EE support (added in v6.3.0), but then I ran into #12838 with combinations of descriptions + single-ref allOfs. This PR apparently solves that.