server(alarms): provider-mode gauge startup baseline; reconcile-lock comment; de-flake monitor test

src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs

@@ -56,8 +56,11 @@ public sealed class GatewayAlarmMonitorProviderModeTests
         await monitor.StartAsync(cts.Token);
         await sessions.WaitForSubscribeAsync(WaitTimeout);
 
-        // Subscribe a live feed reader, drain its first (provider status) message.
+        // Subscribe a live feed reader. Gate emitting the mode-change event until the
+        // reader has consumed its baseline ProviderStatus message, avoiding a race where
+        // the event arrives before the subscriber is registered and draining its snapshot.
         List<AlarmFeedMessage> received = [];
+        TaskCompletionSource baselineReceived = new(TaskCreationOptions.RunContinuationsAsynchronously);
         using CancellationTokenSource streamCts = new();
         Task reader = Task.Run(async () =>
         {
@@ -65,7 +68,15 @@ public sealed class GatewayAlarmMonitorProviderModeTests
             {
                 await foreach (AlarmFeedMessage message in monitor.StreamAsync(null, streamCts.Token))
                 {
-                    lock (received) { received.Add(message); }
+                    lock (received)
+                    {
+                        received.Add(message);
+                        // Signal once the first message (baseline ProviderStatus) has arrived.
+                        if (received.Count == 1)
+                        {
+                            baselineReceived.TrySetResult();
+                        }
+                    }
                 }
             }
             catch (OperationCanceledException)
@@ -74,10 +85,13 @@ public sealed class GatewayAlarmMonitorProviderModeTests
             }
         });
 
+        // Wait for the baseline ProviderStatus to arrive before emitting the mode change,
+        // so the subscriber is registered and the event is not dropped.
+        await baselineReceived.Task.WaitAsync(WaitTimeout);
+
         // Emit the worker event that flips the provider into subtag mode.
         sessions.EmitEvent(new MxEvent
         {
-            Family = MxEventFamily.OnAlarmProviderModeChanged,
             OnAlarmProviderModeChanged = new OnAlarmProviderModeChangedEvent
             {
                 Mode = AlarmProviderMode.Subtag,