OpenAPI: Add snapshot test for nullable reference type arrays

[Youssef1313]

Follow-up to #68250

I realized late that the added integration tests in #68250 mostly covered nullable value types and not nullable reference types. The only test that covered nullable reference types was a unit test in OpenApiSchemaService/OpenApiSchemaService.ParameterSchemas.cs

[Copilot]

Pull request overview

Adds coverage to the OpenAPI integration snapshot tests to ensure nullable reference-type array element nullability (e.g., string?[]) is represented correctly in generated parameter schemas, as a follow-up to #68250.

Changes:

• Adds a new sample endpoint exercising nullable vs non-nullable reference array elements in query parameters.
• Updates OpenAPI integration snapshots (3.0/3.1/3.2) to include the new path and expected items nullability representation.
• Updates the localization invariance snapshot to include the same new path and schema output.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/OpenApi/sample/Endpoints/MapSchemasEndpoints.cs	Adds a new endpoint for string?[] vs string[] query parameters to drive snapshot coverage.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApiDocumentLocalizationTests.VerifyOpenApiDocumentIsInvariant.verified.txt	Updates invariant snapshot with the new endpoint’s expected schema.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_2/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=schemas-by-ref.verified.txt	Updates OpenAPI 3.2 snapshot with the new endpoint’s expected schema.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_1/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=schemas-by-ref.verified.txt	Updates OpenAPI 3.1 snapshot with the new endpoint’s expected schema.
src/OpenApi/test/Microsoft.AspNetCore.OpenApi.Tests/Integration/snapshots/OpenApi3_0/OpenApiDocumentIntegrationTests.VerifyOpenApiDocument_documentName=schemas-by-ref.verified.txt	Updates OpenAPI 3.0 snapshot with the new endpoint’s expected schema using nullable: true on items.

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