Fix flaky GalaxyHierarchyRefreshServiceTests timing race

ExecuteAsync_WhenFirstRefreshThrowsNonCancellationException_DoesNotFault
BackgroundService cancelled the service immediately after StartAsync, so
under parallel load the first RefreshAsync could be skipped (RefreshCallCount
0) and `await executeTask` rethrew TaskCanceledException before the IsFaulted
assertion. The test now waits for a TaskCompletionSource signal that the
first refresh was attempted before cancelling, and uses Task.WhenAny so a
Canceled ExecuteTask does not rethrow. Confirmed stable across full-suite
runs (408/408).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Tests/Galaxy/GalaxyHierarchyRefreshServiceTests.cs

@@ -22,13 +22,23 @@ public sealed class GalaxyHierarchyRefreshServiceTests
         using CancellationTokenSource cts = new();
 
         await service.StartAsync(cts.Token);
+
+        // Wait until the first RefreshAsync has actually been attempted (and
+        // thrown) before cancelling, so cancellation cannot race ahead of the
+        // first-load path under test — this is what made the test flaky under
+        // parallel load.
+        await cache.FirstRefreshAttempted.WaitAsync(TimeSpan.FromSeconds(10));
+
         await cts.CancelAsync();
 
-        // The background loop must have stopped cleanly: ExecuteTask completes
-        // (RanToCompletion or Canceled) rather than faulting on the first refresh.
+        // The background loop must have stopped cleanly: ExecuteTask reaches a
+        // terminal state that is not Faulted (RanToCompletion or Canceled)
+        // rather than faulting on the first refresh. WhenAny is used so a
+        // Canceled task does not rethrow before the IsFaulted assertion.
         Task? executeTask = service.ExecuteTask;
         Assert.NotNull(executeTask);
-        await executeTask;
+        Task completed = await Task.WhenAny(executeTask, Task.Delay(TimeSpan.FromSeconds(10)));
+        Assert.Same(executeTask, completed);
         Assert.False(executeTask.IsFaulted);
         Assert.Equal(1, cache.RefreshCallCount);
 
@@ -49,13 +59,20 @@ public sealed class GalaxyHierarchyRefreshServiceTests
 
     private sealed class ThrowingCache(Exception toThrow) : IGalaxyHierarchyCache
     {
+        private readonly TaskCompletionSource firstRefreshAttempted =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
         public int RefreshCallCount { get; private set; }
 
+        /// <summary>Completes once <see cref="RefreshAsync"/> has been invoked at least once.</summary>
+        public Task FirstRefreshAttempted => firstRefreshAttempted.Task;
+
         public GalaxyHierarchyCacheEntry Current => GalaxyHierarchyCacheEntry.Empty;
 
         public Task RefreshAsync(CancellationToken cancellationToken)
         {
             RefreshCallCount++;
+            firstRefreshAttempted.TrySetResult();
             throw toThrow;
         }