Fix issue 22209: [JAVA-gen] Fix oneOf inheritance: prevent generation of abstract wrapper and ensure real POJO model is produced

[Chhida]

This PR fixes this issue https://github.com/OpenAPITools/openapi-generator/issues/22209

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[Chhida]

Hi, I added a fix related to (https://github.com/OpenAPITools/openapi-generator/issues/22209). Tests and compilation pass locally. Could you please review and let me know if any further changes are needed before merging? Thanks!

cc java technical committee: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)