Kotlin Multiplatform polymorphism with custom discriminator support

[yshrsmz]

related: #14301, #21531

This is essentially a successor of #21531.
I've added discriminator support for kotlin-client multiplatform, as asked in https://github.com/OpenAPITools/openapi-generator/pull/21531/files?w=1#r2276511440.

• added a missing multiplatform check in KotlinClientCodegen#postProcessAllModels(Map)
• updated related templates.
• added kotlin-multiplatform-allOf-discriminator

@dr4ke616 (2018/08) @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @yutaka0m (2020/03) @stefankoppier (2022/06) @e5l (2024/10)

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[wing328]

please review the build failure when you've time

[yshrsmz]

@wing328 thanks, fixed the issue. At least my local bash bin/generate-samples.sh shows no diff.

[wing328]

what about https://github.com/OpenAPITools/openapi-generator/actions/runs/17288142203/job/49073090200?pr=21772?

is it caused by this PR?

[yshrsmz]

@wing328 yes it was caused by this PR, but said samples/client/petstore/kotlin-multiplatform-allOf-discriminator/gradlew is actually committed here: https://github.com/OpenAPITools/openapi-generator/pull/21772/files#diff-892a177c01f704378fb7dc7c996b85d4d1b0678807bb609e240c08f60d7972ba

[yshrsmz]

@wing328

ok found the problem. I named the sample project kotlin-multiplatform-allOff-discriminator(allOff), but tried to reference that project as kotlin-multiplatform-allOf-discriminator(allOf).

While I think it should be allOf, I discovered that the existing samples like kotlin-allOff-discriminator or kotlin-allOff-discriminator-kotlinx-serialization use allOff.

Should I follow existing convention or use allOf?