[BUG] [Kotlin] Sibling properties are ignored

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?

Description

Spec with allOf inheritance and sibling properties is not generated correctly in kotlin.

Version

$ openapi-generator-cli version
5.3.0

OpenAPI declaration file content or url

{
  "openapi": "3.0.1",
  "info": {
    "title": "title",
    "version": "v1"
 },
  "paths": {},
  "components": {
      "schemas": {
          "Generic": {
              "type": "object",
              "properties": {
                  "a": {
                      "type": "string"
                  }
              }
          },
          "Derived": {
              "type": "object",
              "allOf": [
                  {
                      "$ref": "#/components/schemas/Generic"
                  }
              ],
              "properties": {
                  "b": {
                      "type": "string"
                  }
              }
          }
      }
  }
}

Output vs Expected Output

openapi-generator-cli generate -g kotlin -i swagger.json

which results with

data class Derived (
    @Json(name = "a")
    val a: kotlin.String? = null
)

however, I would expect

data class Derived (
    @Json(name = "a")
    override val a: kotlin.String? = null,
    @Json(name = "b")
    val b: kotlin.String? = null
) : Generic

Related issues/PRs

Most related issues (e.g. #4171) use a spec where properties are elements in allOf array. However, Swashbuckle.AspNetCore generates a spec where properties are a sibling object which is a perfectly syntax as claimed by the that lib maintainers.

What's at fault?

Generating java models with

openapi-generator-cli generate -g java -i swagger.json

works perfectly fine with the same spec. It seems that the problem is with the kotlin generator itself.

Reproduce the issue with the supplied OpenAPI declaration and openapi-generator-cli generate -g kotlin -i swagger.json, then compare the generated Derived model with the expected output. Trace the Kotlin generator path responsible for allOf inheritance and sibling properties; done means the generated model includes both a and b, the override, and the Generic inheritance.