Fix flaky test: add sync point in ReadEventsAsync_InStreamingMode_YieldsNewlyWrittenEvents (#1491)

tests/ModelContextProtocol.Tests/Server/DistributedCacheEventStreamStoreTests.cs

@@ -904,10 +904,16 @@ public class DistributedCacheEventStreamStoreTests(ITestOutputHelper testOutputH
         using var cts = CancellationTokenSource.CreateLinkedTokenSource(CancellationToken);
         cts.CancelAfter(TimeSpan.FromSeconds(10));
         var events = new List<SseItem<JsonRpcMessage?>>();
+
+        // Use a TCS as a sync point: set when the reader has confirmed receipt of the first event.
+        // This guarantees the streaming enumerator is definitely active before we write events 2 and 3.
+        var readerStarted = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+
         var readTask = Task.Run(async () =>
         {
             await foreach (var evt in reader.ReadEventsAsync(cts.Token))
             {
+                readerStarted.TrySetResult(true);
                 events.Add(evt);
                 if (events.Count >= 3)
                 {
@@ -916,8 +922,13 @@ public class DistributedCacheEventStreamStoreTests(ITestOutputHelper testOutputH
             }
         }, CancellationToken);
 
-        // Write 3 new events - the reader should pick them up since it's in streaming mode
+        // Write the first event and wait for the reader to confirm it has been received.
+        // This establishes a synchronization point: once readerStarted is signalled, we know
+        // the streaming loop is running and will reliably observe any subsequently written events.
         var event1 = await writer.WriteEventAsync(new SseItem<JsonRpcMessage?>(null), CancellationToken);
+        await readerStarted.Task.WaitAsync(cts.Token);
+
+        // Write the remaining 2 events now that the reader is confirmed active
         var event2 = await writer.WriteEventAsync(new SseItem<JsonRpcMessage?>(null), CancellationToken);
         var event3 = await writer.WriteEventAsync(new SseItem<JsonRpcMessage?>(null), CancellationToken);