[BUG][KOTLIN] Generator outputs Type Mismatch for `object` body param

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

The Kotlin multiplatform generator outputs code with syntax errors due to a type mismatch between the request body and private class created to represent the request body.

    open suspend fun postFoo(body: kotlin.String?): HttpResponse<kotlin.String> { // body is a String

        val localVariableAuthNames = listOf<String>()

        val localVariableBody = PostFooRequest(body) // body passed to request object constructor

        val localVariableQuery = mutableMapOf<String, List<String>>()

        val localVariableHeaders = mutableMapOf<String, String>()

        val localVariableConfig = RequestConfig<kotlin.Any?>(
            RequestMethod.POST,
            "/foo/post",
            query = localVariableQuery,
            headers = localVariableHeaders
        )

        return jsonRequest(
            localVariableConfig,
            localVariableBody,
            localVariableAuthNames
        ).wrap()
    }

    @Serializable
    private class PostFooRequest(val value: Map<kotlin.String, kotlin.String>) { // constructor takes map not string
        @Serializer(PostFooRequest::class)
        companion object : KSerializer<PostFooRequest> {
            private val serializer: KSerializer<Map<kotlin.String, kotlin.String>> = serializer<Map<String, kotlin.String>>()
            override val descriptor = serializer.descriptor
            override fun serialize(encoder: Encoder, obj: PostFooRequest) = serializer.serialize(encoder, obj.value)
            override fun deserialize(decoder: Decoder) = PostFooRequest(serializer.deserialize(decoder))
        }
    }

Yields

Type mismatch: inferred type is String but Map<String, String> was expected

openapi-generator version

5.4.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Request body data type mismatch example
  version: '1.0'
  description: Error example
  contact:
    name: craigberry1
servers:
  - url: 'http://localhost:3000'
paths:
  /foo/post:
    post:
      summary: Get User Info by User ID
      tags: []
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: string
      operationId: post-foo
      description: Post endpoint that takes plain object in request body referenced from model
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CategoriesGetRequest'
    parameters: []
components:
  schemas:
    CategoriesGetRequest:
      title: CategoriesGetRequest
      type: object

Generation Details

openapi-generator generate -i request-body-example.yaml -g kotlin --library multiplatform

Steps to reproduce

1. Create a schema model of type object and no other properties
2. Create an endpoint which accepts this schema in the request body
3. Run api through kotlin/multiplatform generator
4. Observe type mismatch between the request body (kotlin.String) and the constructor args of the private class created (Map<kotlin.String, kotlin.String>)

Related issues/PRs

I have not found any similar issues.

Suggest a fix

I am not familiar enough with the code to suggest a fix. Some questions I would ask are:

• Why is a request body of type object turned into a String?
• I am curious when these serializable request/response templates are invoked vs creating an actual model: https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/serial_wrapper_request_list.mustache

Run the supplied OpenAPI YAML through the kotlin generator with the multiplatform library and inspect the generated request model and method. Start with the Kotlin multiplatform templates, especially modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/serial_wrapper_request_list.mustache; done means the generated client accepts the empty object body without a Kotlin type mismatch.