VSTHRD103 does not flag sync extension method when async alternative exists in the same static class #1568

VSTHRD103 does not produce a diagnostic when calling a synchronous extension method from an async method, even though an *Async alternative is defined in the same static class and is in scope.

Repro

using System.Threading.Tasks;

public interface IExecutable { }

public static class ExecutableExtensions
{
    public static string GetOutput(this IExecutable executable) => "";
    public static Task<string> GetOutputAsync(this IExecutable executable) => Task.FromResult("");
}

class Test
{
    async Task DoWorkAsync()
    {
        IExecutable exec = null!;
        string result = exec.GetOutput(); // ← No VSTHRD103 diagnostic here
    }
}

Expected

VSTHRD103 should flag exec.GetOutput() and suggest await exec.GetOutputAsync().

Actual

No diagnostic is produced.

Notes

Both methods are extension methods on IExecutable, defined in the same static class ExecutableExtensions, with the namespace in scope via using. The analyzer does detect instance method pairs and using static scenarios per the existing test suite, but appears to miss extension method invocations using the standard obj.Method() syntax.

Discovered while converting NUnit tests from JoinableTaskFactory.Run(async () => ...) to native async Task in gitextensions/gitextensions#12987.