feat(openapi): support Nullable Description Attribute on enums

[rdeveen]

Support Nullable Description Attribute on enums.

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Description

Improve OpenApiSchemaService to retrieve DescriptionAttribute from underlying types, enabling correct descriptions for enums and nullable enums. Expand tests to cover these scenarios and verify schema descriptions. Standardize HelixContent formatting in project files.

Fixes #65067

[rdeveen]

Thanks to @desjoerd for his support.

[Copilot]

Pull request overview

This pull request fixes a regression in Microsoft.AspNetCore.OpenApi 10.0.2 where DescriptionAttribute on enum types was not being reflected in the generated OpenAPI schemas. The fix ensures that descriptions are correctly retrieved from the underlying type when dealing with nullable enums.

Changes:

• Modified description attribute retrieval logic to check the ElementType (for nullable value types) before falling back to the Type itself
• Added comprehensive test coverage for nullable enum descriptions and enhanced existing tests to verify enum descriptions
• Standardized HelixContent formatting in project files from multi-line to single-line format

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs	Fixed description attribute retrieval to check ElementType for nullable value types (including enums)
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.RequestBodySchemas.cs	Added new test for nullable enum descriptions, enhanced existing test to verify enum schema descriptions, added SampleEnum definition with description, and applied formatting fixes to lambda expressions
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Microsoft.AspNetCore.OpenApi.Tests.csproj	Standardized HelixContent formatting to single-line
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests.csproj	Standardized HelixContent formatting to single-line