Fix preview 5 regression for OpenApi nullable enums

[Youssef1313]

This regressed in #66228

Previously, we were getting the "null" in the enum array, but in #66228 we started to write the array ourselves and we dropped the null for nullable enums. This makes sure that null is kept.

[Copilot]

Pull request overview

Fixes a regression introduced by the enum naming-policy parameter schema rewrite (PR #66228) where nullable enums no longer included null in their generated OpenAPI enum arrays. This restores the prior behavior while preserving the naming-policy correction for non-body parameters.

Changes:

• Update parameter-schema enum rewriting to append null for nullable enum parameters.
• Add integration snapshot coverage for nullable enum behavior across OpenAPI 3.0/3.1/3.2 for both non-body parameters and JSON request bodies.
• Add new integration test fixture and corresponding snapshots for the nullable enum + global naming policy scenario.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs	Restores null enum entry when rewriting non-body nullable enum parameter schemas.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/OpenApiDocumentEnumNamingPolicyIntegrationTests.cs	Adds integration tests to validate nullable enum behavior with a global naming policy.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_0/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiParameters_NullableEnumWithGlobalNamingPolicy_NonBodySchemaOmitsNullEnumEntry.verified.txt	Snapshot asserting nullable enum parameter schema includes null in enum.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_0/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiRequestBody_NullableEnumWithGlobalNamingPolicy_BodySchemaOmitsNullEnumEntry.verified.txt	Snapshot asserting nullable enum body schema retains null handling and enum entry.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_1/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiParameters_NullableEnumWithGlobalNamingPolicy_NonBodySchemaOmitsNullEnumEntry.verified.txt	Same as above for OpenAPI 3.1.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_1/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiRequestBody_NullableEnumWithGlobalNamingPolicy_BodySchemaOmitsNullEnumEntry.verified.txt	Same as above for OpenAPI 3.1.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_2/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiParameters_NullableEnumWithGlobalNamingPolicy_NonBodySchemaOmitsNullEnumEntry.verified.txt	Same as above for OpenAPI 3.2.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_2/OpenApiDocumentEnumNamingPolicyIntegrationTests.GetOpenApiRequestBody_NullableEnumWithGlobalNamingPolicy_BodySchemaOmitsNullEnumEntry.verified.txt	Same as above for OpenAPI 3.2.