Support nullable array element types in parameter schemas

[snemeckayova]

Previously, an endpoint such as app.MapGet("/", (string?[] inputs) => { }); produced a schema where the array items were encoded as non-nullable. This was assumed to be a runtime limitation, since the nullability of a reference type is not part of the type itself and therefore cannot be recovered from typeof(string[]) while generating the item schema.

However, the annotation is available: NullabilityInfoContext.Create(ParameterInfo) exposes NullabilityInfo.ElementType, which carries the nullability of the array's element type. This PR uses that metadata when building parameter schemas.

Changes:
• TypeExtensions.ShouldApplyNullableArrayElementSchema - resolves the element-type nullability from the parameter's ParameterInfo for array parameters.
• OpenApiSchemaExtensions.MakeArrayItemsNullable - marks array items as nullable. Inline item schemas get null added to their type keyword; item schemas that are references to shared component schemas are wrapped in a oneOf with a null schema instead of being mutated in place, so other usages of the component are unaffected.
• OpenApiDocumentService.GetParametersAsync - applies the above to the generated parameter schema.

Behavior for value-type elements (int?[], Guid?[], DateTime?[], ...) is unchanged, as those were already resolved correctly from the type itself.

Fixes #67844

[Copilot]

Pull request overview

Improves OpenAPI schema generation for minimal API parameters declared as nullable reference-type arrays (e.g., string?[]), ensuring the generated parameter schema correctly represents nullable array items by using NullabilityInfo.ElementType from ParameterInfo.

Changes:

• Updates parameter schema generation to detect nullable array element annotations via NullabilityInfoContext.
• Adds MakeArrayItemsNullable to mark array item schemas nullable, wrapping referenced component item schemas in oneOf to avoid mutating shared components.
• Updates the parameter schema tests to expect nullable items for string?[].

Reviewed changes

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

File	Description
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Services/OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs	Updates expectations so string?[] produces nullable array item schemas.
src/OpenApi/src/Services/OpenApiDocumentService.cs	Reuses a computed parameter schema and applies nullable-array-item adjustment when applicable.
src/OpenApi/src/Extensions/TypeExtensions.cs	Adds detection for nullable array element annotations via ParameterInfo nullability metadata.
src/OpenApi/src/Extensions/OpenApiSchemaExtensions.cs	Adds logic to mark array items nullable, using inline type-flagging or oneOf wrapping for referenced item schemas.

Suppressed comments (1)

src/OpenApi/src/Extensions/OpenApiSchemaExtensions.cs:46

• The else branch (wrapping Items in oneOf to avoid mutating referenced component schemas) isn’t covered by the updated tests. Without coverage, it’s easy to regress behavior for array parameters whose element schema is componentized (e.g., SomePoco?[] where SomePoco is emitted as a #/components/schemas/* reference). Please add a test that asserts the item schema becomes a oneOf: [null, $ref] wrapper and that the underlying component schema remains non-nullable.

    public static bool IsComponentizedSchema(this OpenApiSchema schema, [NotNullWhen(true)] out string? schemaId)
    {
        if(schema.Metadata is not null

[Youssef1313]

• If we don't have an integration test for that already, could we add one?
• Can we ensure to have a test where an API has two parameters, one is Model?[] and the second is Model[]? The goal is to ensure if both are componentized, we still get the correct nullability.

[Youssef1313]

When adding integration tests, let's also make sure we have a test for this when it's part of the API response

[Youssef1313]

For the case of testing a response, I think we should have a similar situation as the originally problematic one where the top-level type is nullable array. So something like schemas.MapGet("/response-array-with-nullable-element", () => TypedResults.Ok(Array.Empty<TestEnum>());

[Youssef1313]

We still have a bug here right?

It's existing bug so it's not a blocker to get this PR merged. But if we merge as-is, please make sure to open an issue so that we track fixing it.

[Youssef1313]

LGTM