Simplify xUnit fix: async Task instead of async void + GCHandle

Replace async void with async Task for the test body, await the Task
directly. Eliminates GCHandle, finished TCS, and async void entirely.

This is cleaner xUnit code but does not fix the intermittent test hang,
which has a separate root cause in the product code.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: ericstj