fix: normalize OAS 3.1 schemas with type:[object,"null"] to set nullable:true correctly

[Picazsoo]

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Summary by cubic

Reverts the OAS 3.1 normalization that converted type: [object, "null"] (and other type arrays including "null") into nullable: true, restoring prior behavior. Backs out the fix related to issue 24139 for further review.

^{Written for commit f741e48. Summary will update on new commits.}