[kotlin] add more tests to cover nonPublicApi, explicitApi options

[wing328]

a follow up pr to #23796

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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Summary by cubic

Add Kotlin client samples and tests for non-discriminator oneOf/anyOf using kotlinx-serialization, covering both explicitApi and nonPublicApi, and update CI to build and verify them. This validates generateOneOfAnyOfWrappers and prevents regressions in the Kotlin generator.

• New Features

  • Added two sample projects: kotlin-oneOf-anyOf-kotlinx-serialization-explicitApi and kotlin-oneOf-anyOf-kotlinx-serialization-nonPublicApi.
  • New configs in bin/configs/ with library=jvm-retrofit2, dateLibrary=java8, enumUnknownDefaultCase=true, serializationLibrary=kotlinx_serialization, and generateOneOfAnyOfWrappers=true (plus explicitApi or nonPublicApi).
  • Included generated models/APIs, adapters, Gradle setup, and tests to exercise oneOf/anyOf wrappers across model, primitive, and array types.

• CI

  • Updated .github/workflows/samples-kotlin-client.yaml to include:
    • samples/client/others/kotlin-oneOf-anyOf-kotlinx-serialization*/**
    • samples/client/others/kotlin-oneOf-discriminator-kotlinx-serialization*/**
  • Ensures push and PR workflows build and check the new samples.

^{Written for commit 506e2ce. Summary will update on new commits. Review in cubic}