fix(kotlin-spring): fix Flow<String> and Flux<T> array return types for reactive generators

[Picazsoo]

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Summary by cubic

Fixes reactive array return types in Kotlin Spring to avoid broken decoding and invalid signatures. Arrays of strings now return List<String>/Set<String> with suspend (not Flow<String>), and declarative Reactor interfaces return Mono<List<T>>/Mono<Set<T>> (not Flux<T>).

• Bug Fixes

  • Coroutines (kotlin-spring): for array-of-string responses, generate suspend + List<String>/Set<String> to avoid Spring WebFlux StringDecoder swallowing JSON arrays (issue 22662).
  • Declarative Reactor (spring-declarative-http-interface): arrays now use Mono<{{returnContainer}}<T>> (e.g., Mono<List<T>>, Mono<Set<T>>), including Mono<ResponseEntity<...>> when enabled; removed reactor.core.publisher.Flux import.
  • Kotlin type handling: unwrap Set/MutableSet like List so returnType is the inner type and returnContainer is Set, fixing cases like Mono<set<kotlin.collections.Set<...>>>.
  • Added x-reactive-array-string-return vendor extension and updated templates to conditionally emit the new signatures.
  • Updated samples to use Mono<List<...>>/Mono<Set<...>> and drop Flux imports.

• Migration

  • Coroutines server: replace Flow<String> with List<String> (or Set<String> for uniqueItems) and mark methods suspend.
  • Declarative Reactor: replace Flux<T> with Mono<List<T>>/Mono<Set<T>> (and Mono<ResponseEntity<...>> when used); update imports.

^{Written for commit a10b418. Summary will update on new commits.}