Ambiguous method call for Task of nullable types

[FNERNST]

Describe the bug
When using the Returns extension method with a Task that may contain a nullable value, the method is ambiguous. The same happens for non primitive nullable types. See the example below which results in a compilation error.

To Reproduce

public interface ITest 
{
  Task<int?> TestAsync();
}

var test = Substitute.For<ITest>();
test.TestAsync().Returns(Task.FromResult(1));

Expected behaviour
The correct Returns method is called.

Environment:

• NSubstitute version: 5.0.0
• NSubstitute.Analyzers version: CSharp 1.0.16
• Platform: .NET Core 7 on macOS

[304NotModified]

I can confirm this issue:

The issue is that Task.FromResult(1); will create a Task<int> and not a Task<int?>