Honor nullable collection elements in OpenAPI schemas

[snemeckayova]

Description

Updates OpenAPI schema generation to preserve nullable annotations for collection elements and dictionary values.
Nullable inline element schemas include null in their type. Nullable componentized elements use a nullable oneOf wrapper around the component reference. The same behavior is applied to model properties, dictionary values, and array response elements, including asynchronous endpoint return types.

Fixes #68322

[Copilot]

🟢 Approval recommended

The changes directly address the described nullability loss, are consistently applied across properties/items/additionalProperties, and are backed by targeted tests plus updated integration snapshots.

Pull request overview

This PR updates ASP.NET Core’s OpenAPI schema generation to correctly preserve C# nullable annotations for collection element schemas (arrays/lists) and dictionary value schemas, including when those element/value schemas are componentized (moved into #/components/schemas/*). This aligns generated documents with expected OpenAPI nullability semantics and fixes the reported regression in #68322.

Changes:

• Propagates “nullable element/value” intent through schema generation via x-is-nullable-property metadata for componentized item/value schemas and resolves them into oneOf nullable wrappers.
• Adds response-specific handling so nullable array element return types (including async returns) produce nullable items schemas.
• Extends test coverage and updates integration snapshots across OpenAPI 3.0/3.1/3.2 outputs.

File summaries

File	Description
src/OpenApi/src/Services/Schemas/OpenApiSchemaService.cs	Ensures nullable wrappers are applied when resolving references for properties, array items, and dictionary additionalProperties.
src/OpenApi/src/Services/OpenApiDocumentService.cs	Applies nullable-item handling for response schemas when the action return type’s array element is nullable.
src/OpenApi/src/Extensions/TypeExtensions.cs	Adds response array-element nullability detection using NullabilityInfoContext and refactors response nullability logic into a shared helper.
src/OpenApi/src/Extensions/JsonNodeSchemaExtensions.cs	Applies nullable annotations to collection element schemas and dictionary value schemas during JsonNode-based schema generation, including componentized cases via metadata.
src/OpenApi/test/.../OpenApiSchemaService.ResponseSchemas.cs	Adds a focused test for nullable componentized array elements in response schemas.
src/OpenApi/test/.../OpenApiSchemaService.PropertySchemas.cs	Adds a focused test for nullable componentized collection elements in request model property schemas.
src/OpenApi/test/.../snapshots/*.verified.txt	Updates expected OpenAPI document snapshots to reflect preserved element/value nullability across versions.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 0
• Review effort level: Lite

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