Do not call ConfigureAwait(false) from within the CustomSynchronizationContext.

Author: David Ellingsworth

src/RestSharp/AsyncHelpers.cs

@@ -97,9 +97,12 @@ public void Run() {
 
             return;
 
+            // This method is only called from within this custom context for the initial task.
             async void PostCallback(object? _) {
                 try {
-                    await _task().ConfigureAwait(false);
+                    // Do not call ConfigureAwait(false) here to ensure all continuations are
+                    // queued on this context, not the thread pool.
+                    await _task();
                 }
                 catch (Exception exception) {
                     _caughtException = ExceptionDispatchInfo.Capture(exception);