[BUG][dart-Dio] Inheritance properties are flattened/missing when using json_serializable

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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

When generating a Dart Dio client with useJsonSerializable=true, models using inheritance (e.g., via allOf or parent/child relationships) fail to generate the correct constructor structure. Specifically, inherited properties are completely removed from the child model's property lists, preventing the generation of a proper super() call in the constructor.

openapi-generator version

OpenAPI declaration file content or url

Generation Details

Steps to reproduce

• Define a parent model and a child model using allOf in the OpenAPI specification.
• Generate code using the dart-dio generator with the following properties: serializationLibrary=json_serializable and useJsonSerializable=true
• Observe that the child model constructor does not include parameters for parent properties and does not call super().

Related issues/PRs

Suggest a fix

The issue lies in how postProcessAllModels handles inheritance in DartDioClientCodegen.java. When inheritance support is active, the generator's internal property separation logic is failing for the Dart-Dio library:

• parentVars (the list for inherited properties) remains empty for child models.
• Inherited properties are incorrectly stripped from the local vars list without being moved to the parentVars list, leading to data loss in the generator's model state.

The fix requires a manual injection and separation of properties within the postProcessAllModels override to ensure the data structures satisfy Dart's constructor requirements.

Start in DartDioClientCodegen.java, especially the postProcessAllModels override and its inheritance property-separation logic. Reproduce with parent and child models joined by allOf using serializationLibrary=json_serializable and useJsonSerializable=true. Done means inherited properties remain available to the child model and its constructor includes them with a super() call.