Fix S3971 FP: Support IAsyncLifetime (xUnit v3)

[dnnr]

Description

Rule S3971 raises a false positive when calling GC.SuppressFinalize in a DisposeAsync method of a class that implements IAsyncLifetime from xUnit. The interface itself inherits IAsyncDisposable, so S3971 should accept this accordingly.

Unlike what I initially assumed, this doesn't seem to be a regression of #3639, but is caused by the intermediate interface. In the attached reproducer, Foo1 is the only case where S3971 is triggered.

Reproducer

class Foo1 : IAsyncLifetime
{
    public ValueTask DisposeAsync()
    {
        GC.SuppressFinalize(this); // <- S3971 complains here
        return ValueTask.CompletedTask;
    }

    public ValueTask InitializeAsync() => throw new NotImplementedException();
}

class Foo2 : IAsyncDisposable
{
    public ValueTask DisposeAsync()
    {
        GC.SuppressFinalize(this);
        return ValueTask.CompletedTask;
    }
}

class Foo3 : IDisposable
{
    public void Dispose()
    {
        GC.SuppressFinalize(this);
    }
}

Product and Version

SonarQube Server v2025.3.1