[Kotlin] Missing underscore prefix for numbers in enum

Description

Kotlin enum class generation is broken for wrong enum convention naming beginning with number.

Swagger-codegen version

2.4.2

Swagger declaration file content or url

"swagger": "2.0"

  TimeFormat:
    type: string
    enum:
      - 12h
      - 24h

Generates this corrupt kotlin file

import com.squareup.moshi.Json

/**
* 
* Values: 12h,24h
*/
enum class TimeFormat(val value: kotlin.String){

    @Json(name = "12h") 12h("12h"),

    @Json(name = "24h") 24h("24h");

}

Swift4 same file correct generation

import Foundation


public enum TimeFormat: String, Codable {
    case _12h = "12h"
    case _24h = "24h"

}

Command line used for Kotlin generation

swagger-codegen generate -l kotlin -i swagger.api.yaml -c swagger-codegen.config.json -o my_directory

Command line used for Swift4 generation

swagger-codegen generate -l swift4 -i swagger.api.yaml -c swagger-codegen.config.json -o my_directory

Suggest a fix/enhancement

Add test for naming conventions. Make strict name conventions rules for enum in .yaml file or for validator. Simple add of underscore is enough.

Start with the Kotlin generator invoked by swagger-codegen generate -l kotlin and trace how enum values from swagger.api.yaml become Kotlin enum members. Add a regression test for numeric enum values such as 12h and 24h, then verify generation produces valid Kotlin identifiers while preserving the original serialized values.