[BUG] [Kotlin] [Gradle Plugin] [Android] Configuration option for mutable/immutable models behaves unexpectedly

Description

I am using the OpenApiGenerator via the Gradle plugin in my Kotlin Android project. I noticed an issue where the configuration value for mutable model attributes is not working as expected.

According to the documentation for the Kotlin generator, a configuration option for mutableModel can be set. The description of this option is simply, "Create mutable models" and the default value is false.
https://github.com/OpenAPITools/openapi-generator/blob/master/docs/generators/kotlin.md

I have noticed that in my particular case, setting any value results in mutable models. The lack of a value all together is what actually results in immutable models.

For example, if I set mutableModel to either true or false, the models generated are made up of all var attributes:

@Parcelize
@JsonClass(generateAdapter = true)
data class UserDto (
    @Json(name = "id")
    var id: kotlin.String,
    @Json(name = "email")
    var email: kotlin.String,
    @Json(name = "isVerified")
    var isVerified: kotlin.Boolean,
    @Json(name = "givenName")
    var givenName: kotlin.String,
    @Json(name = "familyName")
    var familyName: kotlin.String
) : Parcelable

If I leave the configuration value off completely, attributes default to val:

@Parcelize
@JsonClass(generateAdapter = true)
data class UserDto (
    @Json(name = "id")
    val id: kotlin.String,
    @Json(name = "email")
    val email: kotlin.String,
    @Json(name = "isVerified")
    val isVerified: kotlin.Boolean,
    @Json(name = "givenName")
    val givenName: kotlin.String,
    @Json(name = "familyName")
    val familyName: kotlin.String
) : Parcelable

My Configuration

build.gradle

classpath "org.openapitools:openapi-generator-gradle-plugin:5.1.0"

App-level build.gradle:

openApiGenerate {
    generatorName = "kotlin"
    inputSpec = "$projectDir/openapi-spec.json"
    ignoreFileOverride = "$projectDir/.openapi-generator-ignore"
    outputDir = "$buildDir/generated/openapi"
    apiPackage = "com.my.example.api.service"
    modelPackage = "com.my.example.api.model"
    configFile = "$projectDir/openapi-generator-config.json"
}

openapi-generator-config.json

{
  "apiSuffix": "Service",
  "collectionType": "list",
  "enumPropertyNaming": "UPPERCASE",
  "library": "jvm-retrofit2",
  "modelMutable": "false",
  "moshiCodeGen": "true",
  "packageName": "com.my.example.api",
  "parcelizeModels": "true",
  "sortModelPropertiesByRequiredFlag": "true",
  "sortParamsByRequiredFlag": "true",
  "useCoroutines": "true",
  "typeMappings": {
    "java.net.URI": "kotlin.String"
  }
}

Related Tickets

https://github.com/OpenAPITools/openapi-generator/pull/4115
https://github.com/OpenAPITools/openapi-generator/issues/3803

Start with docs/generators/kotlin.md and the Gradle plugin configuration shown in build.gradle and openapi-generator-config.json. Reproduce the generated Kotlin models with the option enabled, disabled, and omitted, then compare the documented mutableModel name with the supplied configuration. Done means true and false produce distinct documented behavior and the configuration documentation matches the implementation.