Fix MergeMany stress test timing for queue-based delivery

The MultiThreadedStressTest asserts immediately after stress observables
complete, but with drain-outside-lock delivery, Edit() returns after
enqueueing while delivery may still be in-flight on another thread.
Add a short delay before checking results to allow in-flight deliveries
to complete.

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

Author: dwcullop

src/DynamicData.Tests/Cache/MergeManyChangeSetsCacheSourceCompareFixture.cs

@@ -118,6 +118,11 @@ IObservable<MarketPrice> AddRemovePrices(Market market, int priceCount, int para
         }
         while (adding);
 
+        // Allow any in-flight notification deliveries to complete before checking results.
+        // With the queue-based drain pattern, Edit() returns after enqueueing but delivery
+        // may still be in progress on another thread. Give the drain a moment to finish.
+        await Task.Delay(250).ConfigureAwait(false);
+
         // Verify the results
         CheckResultContents(_marketCacheResults, priceResults, Market.RatingCompare);
     }