[dart-dio] Fix nullable inline object models

[ShellWen]

What does this PR do?

Fixes nullable handling for OAS 3.1 inline object schemas using the type-array form, e.g. type: [object, null], after they are promoted to inline component models.

This keeps dart-dio fields nullable for schemas like issue #23866.

How was this tested?

mvn -s /tmp/openapi-generator-maven-settings.xml \
  -pl modules/openapi-generator \
  -Dtest=org.openapitools.codegen.dart.dio.DartDioModelTest,org.openapitools.codegen.utils.ModelUtilsTest,org.openapitools.codegen.validations.oas.OpenApiSchemaValidationsTest,org.openapitools.codegen.DefaultCodegenTest \
  test

Result: Tests run: 254, Failures: 0, Errors: 0, Skipped: 0.

Fixes #23866

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Summary by cubic

Fixes nullable handling for OAS 3.1 inline object schemas using type arrays like type: [object, null], so promoted inline models keep nullability. Generated dart-dio models now correctly mark these fields as nullable.

• Bug Fixes
  • Extend ModelUtils.isNullable to detect type-array null and use it across model, property, and parameter generation (including refs).
  • Preserve nullable when promoting inline schemas and creating $refs/components in InlineModelResolver and DefaultCodegen (carry required too).
  • Update OAS 3.1 validation to ignore type-array null and warn only on deprecated nullable/x-nullable; add tests and a 3.1 fixture.

^{Written for commit e1561fe. Summary will update on new commits. Review in cubic}

[Copilot]

Pull request overview

Fixes nullability propagation for OpenAPI 3.1 inline object schemas declared with type-array unions (e.g. type: [object, "null"]) after they’re promoted to inline component models, ensuring generators (notably dart-dio) keep the field nullable.

Changes:

• Extend ModelUtils.isNullable to treat schema.types containing "null" as nullable.
• Preserve/propagate nullability when inline schemas are promoted to $ref via InlineModelResolver, and use ModelUtils.isNullable more consistently in DefaultCodegen (models, properties, parameters).
• Add regression coverage across dart-dio, DefaultCodegen, ModelUtils, and OAS schema validation recommendations, plus a new repro spec fixture.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
modules/openapi-generator/src/test/resources/3_1/dart-dio/issue_23866.yaml	Adds the OAS 3.1 repro spec fixture used by new/updated tests.
modules/openapi-generator/src/test/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidationsTest.java	Adds test ensuring type-array null doesn’t trigger the deprecated nullable recommendation.
modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java	Adds unit test verifying ModelUtils.isNullable detects type-array null.
modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java	Adds regression test asserting CodegenProperty.isNullable for type-array null inline object properties.
modules/openapi-generator/src/test/java/org/openapitools/codegen/dart/dio/DartDioModelTest.java	Adds dart-dio-specific regression test asserting nullability is preserved for promoted inline object models.
modules/openapi-generator/src/main/java/org/openapitools/codegen/validations/oas/OpenApiSchemaValidations.java	Refines nullable-attribute recommendation to only trigger on actual nullable/x-nullable, not on type-array null.
modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java	Updates isNullable implementation and Javadoc to include OAS 3.1 type-array null handling.
modules/openapi-generator/src/main/java/org/openapitools/codegen/InlineModelResolver.java	Ensures nullability survives inline schema promotion to $ref by setting nullable on generated ref schemas when appropriate.
modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java	Switches nullability detection to ModelUtils.isNullable across model/property/parameter paths and adds a helper to avoid overwriting derived nullability unless explicitly marked.

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[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}