Fix all ofs with nullable properties in OpenAPI 3.1 specs - #25064

This PR fixes an error where using all ofs with nullable properties in an OpenAPI spec causes the nullability to get lost.
For an example spec see modules/openapi-generator/src/test/resources/3_1/allof-nullable-type-array.yaml

In this example, id should be nullable in both Base and Derived, but in the generated code (e.g. using the php-nextgen) generator, it is only nullable for Base. That happens because the normalizer turns the 3.1 spec's types: ['string', 'null'] into type: string and nullable: true, but AnnotationsUtils.clone parses the spec as 3.1 where that property doesn't exist and therefore skips it.

This should fix the following issues as well:

• [openapi spec 3.1.0][python-flask generator] Nullable properties not propagated to child models #19619 (python-flask)
• [BUG][typescript-axios] Allof keyword makes nullable fields become non-nullable #21065 (typescript-axios)
• BUG][csharp][generichost] Nullable value-type properties dropped to non-nullable when inherited via allOf, causing JsonException on null #23720 (csharp)

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Summary by cubic

Fixes nullability being lost on properties merged into models via allOf in OpenAPI 3.1 specs.

cloneSchema now restores the nullable flag on the clone and at every nested level (properties, array items, additionalProperties, not, and composed sub-schemas), since the OAS 3.1 serializer drops it. The normalizer rewrites type: [<type>, "null"] into nullable: true, so the flag must survive cloning. Resolves related generator issues for python-flask, typescript-axios, and csharp.

^{Written for commit c7a1933. Summary will update on new commits.}