Fix duplicate schema component for nullable unions in OpenAPI

[DeagleGross]

Problem

A nullable union property duplicated its case type component — OneOrManyBeast referenced OneOrManyBeastBeast instead of Beast, even though the array branch referenced Beast correctly:

"OneOrManyBeast": {
  "anyOf": [
    { "$ref": "#/components/schemas/OneOrManyBeastBeast" },
    { "type": "array", "items": { "$ref": "#/components/schemas/Beast" } }
  ]
},
"OneOrManyBeastBeast": { ... identical to Beast ... }

Cause

Unions are value types, so MyUnion? surfaces as Nullable<MyUnion> whose JsonTypeInfo.Kind is None, not Union. The x-schema-is-union marker was never written, so the existing union carve-out in ResolveReferenceForSchema didn't apply.

The problem was nullability of the union type.

Fix

Unwrap Nullable<> before the Kind check, mirroring GetSchemaReferenceId. Uses TryGetTypeInfo so a resolver missing the underlying type degrades instead of throwing.

Fixes #68653

[Youssef1313]

Haven't debugged through it, but the change looks reasonable on a high level.

[Copilot]

Pull request overview

Fixes an OpenAPI schema component duplication that occurs when a nullable C# union is used in a nested position and one union branch is a polymorphic base type. The change ensures nullable unions are still detected as unions during schema export so reference resolution can reuse the existing polymorphic component rather than generating a duplicate.

Changes:

• Unwraps Nullable<T> when determining whether to write the x-schema-is-union marker during schema export.
• Adds shared polymorphic “Beast” union types plus targeted unit tests covering direct, nested, nullable, and non-nullable union scenarios.
• Updates sample endpoint and integration snapshots to include the new unions/beasts scenario and validate generated schemas.

Show a summary per file

File	Description
src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs	Unwraps Nullable<> before checking JsonTypeInfo.Kind so nullable unions are still marked with x-schema-is-union.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Shared/SharedTypes.Unions.cs	Adds a polymorphic base type (Beast) and a union (OneOrManyBeast) plus containers to reproduce/validate the duplication scenario.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.UnionSchemas.cs	Adds unit tests asserting both union branches reference the polymorphic base component and that no duplicate component is generated.
src/OpenApi/sample/Endpoints/MapUnionsEndpoints.cs	Adds a sample endpoint exercising a nullable union with a polymorphic case type.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApiDocumentLocalizationTests.VerifyOpenApiDocumentIsInvariant.verified.txt	Updates invariant snapshot to include the new /unions/beasts path and related schemas.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_0/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=unions.verified.txt	Updates OpenAPI 3.0 unions snapshot for the new beasts scenario.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_1/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=unions.verified.txt	Updates OpenAPI 3.1 unions snapshot for the new beasts scenario.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_2/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=unions.verified.txt	Updates OpenAPI 3.2 unions snapshot for the new beasts scenario.

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• Files reviewed: 8/8 changed files
• Comments generated: 0
• Review effort level: Lite