test: run async UI test bodies to completion

Avalonia's Dispatch(Func<Task>) returns at the first real await, so the
assertions after it never ran and async ViewModel tests passed vacuously.
UiSession.Run now drives the body to completion while pumping the
dispatcher. The test profile has no checkpoints, since the fake chains use
tiny heights that the real mainnet checkpoints would try to anchor.

tests/PalladiumWallet.Tests/App/HeadlessSetup.cs

@@ -22,7 +22,23 @@ internal static class UiSession
     private static readonly Lazy<HeadlessUnitTestSession> Session =
         new(() => HeadlessUnitTestSession.StartNew(typeof(TestAppBuilder)));
 
-    public static Task Run(Func<Task> body) => Session.Value.Dispatch(body, CancellationToken.None);
+    // Avalonia's Dispatch(Func<Task>) returns as soon as the body reaches its first real await:
+    // everything after it (including the assertions) would silently never run and the test would
+    // pass vacuously. So the async body is driven to completion here, pumping the dispatcher on
+    // the UI thread (the continuations of its awaits are posted there), with a hard time limit.
+    public static Task Run(Func<Task> body) => Session.Value.Dispatch(() =>
+    {
+        var task = body();
+        var deadline = DateTime.UtcNow + TimeSpan.FromMinutes(2);
+        while (!task.IsCompleted)
+        {
+            if (DateTime.UtcNow > deadline)
+                throw new TimeoutException("Async UI test body did not complete within 2 minutes.");
+            Avalonia.Threading.Dispatcher.UIThread.RunJobs();
+            Thread.Sleep(1);
+        }
+        task.GetAwaiter().GetResult();
+    }, CancellationToken.None);
 
     public static Task Run(Action body) =>
         Session.Value.Dispatch(() => { body(); return Task.CompletedTask; }, CancellationToken.None);