[BUG] [kotlin-spring] in reactive mode Flow<String> is not the same as Mono<List<String>>

Description

The generator triggers an unfortunate behavior in Spring when the response of an operations is an array of plain strings. in that case the generator produces Flow which spring will assume to be raw JSON and concatenate.

See this issue: https://github.com/spring-projects/spring-framework/issues/20807

openapi-generator version

5.2.0

OpenAPI declaration file content or url

openapi: 3.0.0
paths:
  /itu/details:
    get:
      operationId: someOp
      responses:
        '200':
          content:
            application/json:
              schema:
                type: array
                items:
                  type: string

Generation Details

The generator is kotlin-spring with delegatePattern = false. The generated operation method in the controller interface will have return type of Flow<String>, however according to the issue linked about this is not equivalent to List.

So instead of:

fun someOp(): Flow<String>

it should generate something like

suspend fun someOp(): List<String>

as suggested by the issues. This is only necessary for a few specific "low-level" types like String and ByteArray.

Start with the kotlin-spring generator and the provided OpenAPI declaration, using reactive mode with delegatePattern=false. Inspect how the generated controller interface chooses Flow for an array of strings and compare it with Spring issue 20807. Done means the generated signature uses a suitable suspend function and List for the affected low-level types, with coverage for the reproduction.