[JAVA] [SPRING] Handle list of String special case in reactive generator

[TMRGZ]

Related: #24077

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Summary by cubic

Fixes reactive Spring return types for array of string responses by returning Mono<List> or Mono<Set> instead of Flux. Applies to spring-boot and spring-http-interface libraries and aligns generated code with the OpenAPI schema.

• Bug Fixes
  • Added x-reactive-return-except-list-of-string vendor extension.
  • SpringCodegen sets the extension for array responses with base type string in reactive mode.
  • Updated responseType.mustache to emit Mono<ResponseEntity<List<String>>>/Mono<ResponseEntity<Set<String>>> (or without ResponseEntity) instead of Flux<String>.
  • Added tests for spring-boot and spring-http-interface, with and without useResponseEntity, verifying Mono<List<String>>/Mono<Set<String>> signatures.

^{Written for commit b07ba00. Summary will update on new commits.}

[TMRGZ]

The Kotlin Spring generator could need a similar fix but I am not used to code in Kotlin, feel free to add it to this PR

[Picazsoo]

I only reviewed on mobile so far, but do I understand correctly that you are hiding the 'now correct' behavior behind a config option and by default it will still produce the incorrect code?

I would expect it to be either

• a) just a straightforward fix with no fallback. If someone depends on the wrong behavior, they should go and fix their code. They will be probably forced to anyway as the code will not compile after upgrading...
• b) inverted behavior - default to the fix and allow custom config to get the 'wrong' behavior back.

As I mentioned this is based on a very quick skimming of the changes in the PR.

Note: I am not in any way a maintainer of this project - just providing my proverbial 2 cents of feedback (-: