Class models instead of Dart enums

[poka-IT]

Description of the bug

When generating Dart code from OpenAPI specifications, enum types are generated as class models instead of proper Dart enums. This makes the code less idiomatic and harder to use with Dart's pattern matching and switch expressions.

Steps to reproduce

• Create an OpenAPI spec with an enum definition
• Run the generator with Dart output
• Check the generated files

Annotation used

I'm using CLI tool

Expected behavior

The generator should create proper Dart enums using the enum keyword for OpenAPI enum definitions.

Library version

6.0.0

Flutter version

3.24

Flutter channel

stable