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 😮.