[BUG][aspnetcore] Duplicated enum models

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

The issue was fixed in #3622 based on isModel attribute. If the enum has isModel set to true, then the enum isn't generated in objects it's referenced. However, looks like how isModel is set was changed in #3855 . Now enums no longer have isModel set.

openapi-generator version

4.2.0 master

OpenAPI declaration file content or url

openapi: 3.0.2
info:
  title: Test
  version: 0.1.0
servers:
  - url: http://localhost
paths:
  /:
    post:
      operationId: test
      responses:
        200:
          description: test
components:
  schemas:
    TestObject:
      type: object
      properties:
        enumInObject:
          $ref: "#/components/schemas/TestEnum"
    TestEnum:
      type: string
      enum:
        - value1
        - value2

Command line used for generation

openapi-generator generate -g aspnetcore

Steps to reproduce

1. generate
2. TestEnum is generated both in TestEnum.cs and TestObject.cs

Related issues/PRs

#3622, #3855

Suggest a fix

Is there any other way to tell if an enum is referenced or inlined?

Reproduce the duplication with the openapi-generator generate -g aspnetcore command and the YAML declaration in the issue. Review the behavior and changes described in #3622 and #3855, then verify that the generated output contains TestEnum only once, rather than in both TestEnum.cs and TestObject.cs.