#24749 Update kotlin workflow to test new samples

a follow up pr to https://github.com/OpenAPITools/openapi-generator/pull/24725/

PR checklist

• Read the contribution guidelines.
• 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.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Expands Kotlin samples CI to include two Petstore allOf special-discriminator variants and aligns their docs with current generator output. This increases coverage for kotlinx-serialization and multiplatform without changing behavior.

• Adds samples/client/petstore/kotlin-allOf-special-discriminator-kotlinx-serialization and samples/client/petstore/kotlin-multiplatform-allOf-special-discriminator to the matrix in .github/workflows/samples-kotlin-client.yaml.
• Updates docs/BirdApi.md in both samples to fix parameter table order; expect a minor CI time increase; no generator or runtime behavior changes.