[BUG] [Dart] Enums with numeric constant values will generate syntatically incorrect Dart code

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

Numeric enums, such as:

    DeviceStatus:
      required:
        - Online
      type: object
      properties:
        Online:
          type: integer
          description: |
            0 Unknown 1 Online 2 Offline
          default: 0
          enum:
            - 0
            - 1
            - 2

will generate uncompilable dart files:

  /// Returns a new [DeviceStatus] instance.
  DeviceStatus({
    this.online = const DeviceStatusOnlineEnum._(DeviceStatusOnlineEnum.number0),
  });

  DeviceStatusOnlineEnum? decode(dynamic data, {bool allowNull = true}) {
    if (data != null) {
      switch (data.toString()) {
        case 0: return DeviceStatusOnlineEnum.number0;
        case 1: return DeviceStatusOnlineEnum.number1;
        case 2: return DeviceStatusOnlineEnum.number2;
        default:
          if (!allowNull) {
            throw ArgumentError('Unknown enum value to decode: $data');
          }
      }
    }
    return null;
  }

openapi-generator version

7.3.0-snapshot

Generation Details

Steps to reproduce

No special reproduction steps, just generate a regular dart sdk using any numeric enum definition like from above

java -jar openapi-generator-cli.jar generate -i .\errors.yaml -g dart -o with_errors

Start by reproducing the issue with the numeric enum definition and the provided openapi-generator CLI command using the dart generator. Inspect the generated Dart files shown in the report; done means the generated SDK compiles and correctly handles numeric enum defaults and decoding without syntactically invalid cases.