[BUG][dart-dio] OAS 3.1 type: ["object", "null"] on inline object schema produces non-nullable Dart field

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 an OpenAPI 3.1 schema marks an inline object property as nullable via the array-form type: ["object", "null"], the dart-dio generator produces a non-nullable Dart field. The same null intent in other shapes is handled correctly:

The last row is the bug. When the server actually returns data: null (which the spec explicitly allows), dio fails to deserialize and throws a DioException, which downstream callers misinterpret as a business error.

openapi-generator version

7.22.0 (via the Dart openapi_generator package).

OpenAPI declaration file content or url

Minimal repro:

openapi: 3.1.0
info:
  title: Repro
  version: "1.0"
paths:
  /login:
    post:
      operationId: login
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/EnvelopeNullableLoginResponse"
components:
  schemas:
    EnvelopeNullableLoginResponse:
      type: object
      required: [code, data, message]
      properties:
        code:
          type: integer
        data:
          # this is the field that triggers the bug
          type:
            - object
            - "null"
          description: Business data; null on failure
          required:
            - token
          properties:
            token:
              type: string
        message:
          type: string

Generation Details

openapi-generator-cli generate -i spec.yaml -g dart-dio -o out --skip-validate-spec

No custom additionalProperties needed to reproduce. serializationLibrary set to either json_serializable or the default built_value shows the same Dart field type.

Steps to reproduce

1. Save the spec above as spec.yaml.

2. Run the generation command.

3. Open out/lib/src/model/envelope_nullable_login_response.dart.

4. Observe the data field:

   @JsonKey(name: r'data', required: true, includeIfNull: true)
   final EnvelopeNullableLoginResponseData data;   // expected: ...Data? data
   

5. The synthesized inline class is generated correctly on its own — the bug is only that the outer field's type isn't suffixed with ?.

Related issues/PRs

• #21565 — Dart-dio ignoring nullable references (nullableReferenceTypes: true). Overlapping symptom (nullable not honored), different trigger ($ref siblings, not array-form type). Both reduce to: dart-dio doesn't propagate isNullable to the field accessor in every code path.
• #21510 — Dart incorrect nullable when required. Closely related — it's the OAS 3.0 nullable: true + required analog of this issue. The fix likely needs to cover both shapes.
• #20827 — Same class of bug in the Java/Spring generator with type: [string, "null"], suggesting isNullable propagation has language-agnostic gaps for OAS 3.1 array-form null.

Suggest a fix

Best guess from reading the related issues: when the dart-dio templates compute the field type for an object whose schema has an inline type: [..., "null"] array form, the isNullable flag is not making it through to the place that emits the ? suffix. The detection of the synthesized inline class itself does work (the model file is generated), so the gap is between schema-level nullability detected and field-level accessor type rendered.

Start with the dart-dio generator's nullable field-type handling and reproduce the issue using the provided OpenAPI declaration and generation command. Inspect out/lib/src/model/envelope_nullable_login_response.dart, compare the inline data field with the generated synthesized inline class and the nullable cases listed, and consider the issue done when data is emitted as a nullable Dart field for both supported serialization libraries.