[KOTLIN-CLIENT] fix Parent interface class having incorrect collection property type List instead of Set

[Picazsoo]

fixes #22696

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.
  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.
• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR solves a reported issue, reference it using GitHub's linking syntax (e.g., having "fixes #123" present in the PR description)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request. Tagging: @karismann (2019/03) @Zomzog (2019/04) @andrewemery (2019/10) @4brunu (2019/11) @stefankoppier (2022/06) @e5l (2024/10)

---

Summary by cubic

Fixes #22696 by generating Set/MutableSet for Kotlin client interface and data class properties when uniqueItems=true, replacing incorrect List types.

• Bug Fixes

  • Updated interface_req_var.mustache, interface_opt_var.mustache, data_class_req_var.mustache, and data_class_opt_var.mustache to emit kotlin.collections.Set or MutableSet for uniqueItems arrays.
  • Non-unique arrays still use List/MutableList; non-list arrays continue to use Array.

• Refactors

  • Simplified mustache logic to reduce repetition by selecting mutability once and switching between Set/List based on uniqueItems.

^{Written for commit 4bf5a0e. Summary will update on new commits.}

[Picazsoo]

I shall probably add some unit test to prevent regression...

[Picazsoo]

I shall probably add some unit test to prevent regression...

Done - I added tests for kotlin-client (where it was broken) and kotlin-spring (where it was ok, but just to prevent regression...).