[release/10.0-rc1] Model nullable types using oneOf in OpenAPI schema

[github-actions]

Backport of #63301 to release/10.0-rc1

/cc @captainsafia

Model nullable types using oneOf in OpenAPI schema

Add oneOf support for nullable complex types in OpenAPI schemas

Description

This change improves OpenAPI schema generation for nullable types by using the oneOf pattern instead of the nullable property for complex types and collections. The implementation:

• Uses oneOf with null and the actual type schema for nullable complex types in request/response schemas
• Implements proper nullability detection for parameters, properties, and return types using reflection and NullabilityInfoContext
• Prunes null types from componentized schemas to avoid duplication

Fixes #63301

Customer Impact

This change provides more accurate OpenAPI schema generation for nullable types, which improves:

• Client code generation quality for nullable reference types and complex objects
• Compliance with OpenAPI 3.1 specifications for nullable type handling
• Developer experience when working with nullable APIs in strongly-typed languages

Regression?

• Yes
• No

Risk

• High
• Medium
• Low

This is a low-risk change as it's a backport of an already-tested feature that improves schema generation without breaking existing functionality. The changes are isolated to OpenAPI schema generation logic and include new unit and integration tests for coverage. The implementation does consist of a breaking behavioral change so we'd like to get it in for RC1.

Verification

• Manual (required)
• Automated

Packaging changes reviewed?

• Yes
• No
• N/A

[dotnet-policy-service]

Hi @@github-actions[bot]. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed.