[KOTLIN-CLIENT] fix issue #22696 (Parent interface class has incorrect collection property type List instead of Set)

[Picazsoo]

fixes #22696

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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...).