fix(kotlin-spring): fix Flow<String> and Flux<T> array return types f…

[Picazsoo]

…or reactive generators

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Summary by cubic

Fixes reactive array return types in the Kotlin Spring generators to avoid broken decoding and unify API shapes. Arrays of strings now return List<String> with suspend (not Flow<String>), and declarative Reactor interfaces return Mono<List<T>> (not Flux<T>).

• Bug Fixes

  • Coroutines (kotlin-spring): for array-of-string responses, generate suspend + List<String> to avoid StringDecoder swallowing JSON arrays in Spring WebFlux (issue 22662).
  • Declarative Reactor (spring-declarative-http-interface): array responses now use Mono<{{returnContainer}}<T>> (i.e., Mono<List<T>>), including Mono<ResponseEntity<List<T>>> when enabled; drop reactor.core.publisher.Flux import.
  • Added x-reactive-array-string-return vendor extension and updated templates to conditionally emit the new signatures.

• Migration

  • Coroutines server: update implementations and callers from Flow<String> to List<String> and mark methods as suspend where applicable.
  • Declarative Reactor: replace Flux<T> returns with Mono<List<T>> (and Mono<ResponseEntity<List<T>>> when used); update imports and tests.

^{Written for commit 857e8b1. Summary will update on new commits.}