OpenAPI: Prune null from enum/type in componentized schemas

[desjoerd]

OpenAPI: Prune null from enum/type in componentized schemas

Improve handling of nullable enums and componentized types in OpenAPI schema generation.

• Remove "null" from type/enum arrays and use nullable/oneOf for nullability.
• Add tests to verify correct schema references and nullability for parameters and request bodies.

Description

Enums will emit null as a valid option when they are Nullable, this works well when they are inlined, but when it becomes a component it can also be used in a Non-Nullable context. The best way to solve this is to make it never nullable and use "OneOf" with references to add the null value as an option.

Fixes #65017

[Copilot]

Pull request overview

This PR improves OpenAPI schema generation for nullable enums (and componentized schemas) by ensuring null is not emitted as an enum value/type in shared component schemas, and by validating correct nullability handling via additional tests.

Changes:

• Added test coverage for nullable enum request bodies to ensure nullability is represented via oneOf (including a null schema) while keeping the referenced enum component non-nullable.
• Added regression test to ensure enum component schemas don’t gain null enum values based on parameter ordering.
• Updated JSON-schema post-processing to prune null from componentized schema type/enum representations.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.RequestBodySchemas.cs	Adds a request body test asserting nullable enum properties are represented via oneOf + $ref, and the enum component does not include null.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs	Adds a regression test ensuring parameter ordering doesn’t cause null to appear in componentized enum schemas.
src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs	Adjusts pruning logic for componentized schemas to remove null from type/enum structures.

[desjoerd]

@captainsafia another ordering issue around nullable and OneOf. Maybe this can be considered for a portback to release/10 (10.0.4).

[martincostello]

@desjoerd FYI Safia is no longer on the .NET team.

[desjoerd]

I missed that 😮.

[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
To make sure no conflicting changes have occurred, please rerun validation before merging. You can do this by leaving an /azp run comment here (requires commit rights), or by simply closing and reopening.