[BUG][dart-dio] Avoid depending on `Equatable` implementation files

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

With equalityCheckMethod: equatable, an implementation file (package:equatable/src/equatable_utils.dart) is imported and the internal equals and mapPropsToHashCode functions are used.

equals was recently removed in Equatable v2.0.6 and then reverted in v2.0.7.

Suggest a fix

Would it be possible to use Equatable through the public interface instead of depending on internal code that may break again in the future?

Start with modules/openapi-generator/src/main/resources/dart/libraries/dio/serialization/json_serializable/class.mustache, especially the Equatable-related import and generated usage. Check how the template currently uses package:equatable/src/equatable_utils.dart, then verify that generated Dart Dio models rely only on Equatable's public interface and no implementation file. Done means the internal import and internal function dependencies are removed without breaking generated equality behavior.