fix(siteeventlog): suppress snapshot-resync alarm re-emit + coverage + hardening (review)

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs

@@ -82,6 +82,18 @@ public class StoreAndForwardService
     /// recognisable instead of an unattributable empty string.
     /// </summary>
     public const string UnknownSiteSentinel = "$unknown-site";
+
+    /// <summary>
+    /// M1.7: the detail-string prefix written by <see cref="EnqueueAsync"/>
+    /// when an immediate forward attempt throws and the message is buffered for
+    /// the retry sweep. <see cref="EmitSiteEvent"/> matches on this same prefix
+    /// to distinguish a forward <i>failure</i> (logged) from a routine
+    /// no-handler enqueue (not logged), so both the construction site and the
+    /// check reference this single constant rather than duplicating the
+    /// literal — keeping the two ends from drifting apart.
+    /// </summary>
+    private const string BufferedForRetryDetailPrefix = "Buffered for retry";
+
     private Timer? _retryTimer;
     private int _retryInProgress;
 
@@ -261,11 +273,12 @@ public class StoreAndForwardService
         if (category == StoreAndForwardCategory.Notification)
         {
             // Spec: log only forward-failure (the immediate forward threw and the
-            // notification was buffered for retry — detail "Buffered for retry:")
-            // and park. A routine "No handler registered, buffered" enqueue and a
-            // forward-success "Delivered" are deliberately NOT logged.
+            // notification was buffered for retry — detail prefixed
+            // BufferedForRetryDetailPrefix) and park. A routine "No handler
+            // registered, buffered" enqueue and a forward-success "Delivered"
+            // are deliberately NOT logged.
             var isForwardFailure = action == "Queued"
-                && detail.StartsWith("Buffered for retry", StringComparison.Ordinal);
+                && detail.StartsWith(BufferedForRetryDetailPrefix, StringComparison.Ordinal);
             if (!isForwardFailure && action != "Parked")
             {
                 return;
@@ -536,7 +549,7 @@ public class StoreAndForwardService
                 message.LastError = ex.Message;
                 await BufferAsync(message);
 
-                RaiseActivity("Queued", category, $"Buffered for retry: {target} ({ex.Message})");
+                RaiseActivity("Queued", category, $"{BufferedForRetryDetailPrefix}: {target} ({ex.Message})");
                 return new StoreAndForwardResult(true, message.Id, true);
             }
         }
@@ -553,7 +566,7 @@ public class StoreAndForwardService
         await BufferAsync(message);
         RaiseActivity("Queued", category, attemptImmediateDelivery
             ? $"No handler registered, buffered: {target}"
-            : $"Buffered for retry: {target}");
+            : $"{BufferedForRetryDetailPrefix}: {target}");
         return new StoreAndForwardResult(true, message.Id, true);
     }