[BUG] [JAVA/KOTLIN] [SPRING] A reactive controller that returns a JSON array of strings does not generate an object serializable by Jackson

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When generating a reactive controller that should return a JSON array of strings should generate a method that returns Mono<ResponseEntity<List> or Mono<ResponseEntity<Set> depending on the uniqueItems flag instead of Flux.

Related Spring documentation: https://docs.spring.io/spring-framework/reference/web/webflux/reactive-spring.html#webflux-codecs-jackson

Related Stackoverflow question: https://stackoverflow.com/questions/48421597/returning-fluxstring-from-spring-webflux-returns-one-string-instead-of-array-o

openapi-generator version

7.7.0

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: OpenAPI Petstore
  description: "sample to get all user id as string list and string set"
  version: 1.0.0
tags: []
paths:
  /users/id?type=set:
    get:
      operationId: getUserIdSet
      tags:
        - users
      responses:
        '200':
          description: response
          content:
            application/json:
              schema:
                type: array
                "uniqueItems": true
                "items":
                  "type": "string"

  /users/id?type=list:
    get:
      operationId: getUserIdList
      tags:
        - users
      responses:
        '200':
          description: response
          content:
            application/json:
              schema:
                type: array
                "uniqueItems": false
                "items":
                  "type": "string"

Generation Details

Maven settings used:

<configuration>
    <inputSpec>${project.basedir}/api/api.yaml</inputSpec>
    <generatorName>spring</generatorName>
    <generateApis>true</generateApis>
    <generateModels>true</generateModels>
    <!-- configuration -->
    <configOptions>
       <interfaceOnly>true</interfaceOnly>
       <sourceFolder>/src/main/java</sourceFolder>
       <useSpringBoot3>true</useSpringBoot3>
       <reactive>true</reactive>
     </configOptions>
 </configuration>

Steps to reproduce

1. Set up project
2. mvn clean compile

Related issues/PRs

Related but affecting the Java client
https://github.com/OpenAPITools/openapi-generator/pull/16326

Suggest a fix

In case there there is an endpoint with application/json content type, create an exception to generate a collected collection of strings, the Kotlin Spring generator is affected too

Start with the Spring generator using the supplied OpenAPI declaration and reactive configuration, then run mvn clean compile to inspect the generated controller signatures. Done means JSON array responses use Mono<ResponseEntity<List>> or Mono<ResponseEntity<Set>> according to uniqueItems, including for the Kotlin Spring generator.