[BUG] [dart-dio] Enum properties with "default" values create compile errors

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

For a schema that has an Enum property with a "default" value, the generated result has compile errors. This does not happen with string properties that have default values.

Minimal api.json:

https://gist.github.com/mrohlof-protofy/f35cab420710d1ce696c28a584285099

Call:

openapi-generator generate -g dart-dio -i api.json -o ./
dart run build_runner build --delete-conflicting-outputs

Result:

...
27 |  static void _defaults(MyActionBuilder b) => b
28 |      ..executionMode = const ._(ExecutionMode.INSTANT);
...

Issue:

my_action.dart:28:31: Expected an identifier.

Expected Result:

...
27 |  static void _defaults(MyActionBuilder b) => b
28 |      ..executionMode = ExecutionMode.INSTANT;
...

openapi-generator version

7.10.0

OpenAPI declaration file content or url

Minimal example:
https://gist.github.com/mrohlof-protofy/f35cab420710d1ce696c28a584285099

Generation Details

Steps to reproduce

openapi-generator generate -g dart-dio -i api.json -o ./
dart run build_runner build --delete-conflicting-outputs

Related issues/PRs

Suggest a fix

Defaults

Start by running the reported openapi-generator dart-dio command with the linked minimal api.json, then inspect the generated my_action.dart around the enum default assignment. Run dart run build_runner build --delete-conflicting-outputs and verify that enum defaults compile without the invalid ._(...) syntax.