[kotlin-client][kotlin-spring] Fix duplicate discriminator serialization with Jackson used as serialization library

[dgenchev-uniper]

fixes #21733

@dr4ke616 @karismann @Zomzog @andrewemery @4brunu @yutaka0m @stefankoppier @e5l

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[wing328]

https://github.com/OpenAPITools/openapi-generator/actions/runs/16920754046/job/47973874871?pr=21734

please update the samples to fix the build failure when you've time

[dgenchev-uniper]

https://github.com/OpenAPITools/openapi-generator/actions/runs/16920754046/job/47973874871?pr=21734

please update the samples to fix the build failure when you've time

Sorry, didn't see that one. Samples are now updated.