fix(dcl): broadcast SnapshotComplete sentinel to all alarm subscribers

The MxGateway alarm mapper emits the SnapshotComplete framing sentinel with
empty SourceReference/SourceObjectReference. HandleAlarmTransitionReceived
routed every transition by prefix match against the subscriber's source, so
the empty-ref sentinel ('' .StartsWith("<src>.") == false) was dropped for
any specific source. The NativeAlarmActor buffers snapshot conditions and only
flushes them on SnapshotComplete, so statically-active native alarms delivered
only in the initial snapshot (no later live transition) never surfaced.

Broadcast the SnapshotComplete sentinel to all alarm subscribers (bypassing the
source match + type filter) so each NativeAlarmActor's snapshot swap completes.
Adds a regression test using the real empty-ref sentinel against a specific
(prefix) source.

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs

@@ -1570,6 +1570,26 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
 
         var transition = msg.Transition;
         var notified = new HashSet<IActorRef>();
+
+        // A SnapshotComplete is a connection-wide framing sentinel, not a real
+        // condition: the mapper emits it with an empty SourceReference /
+        // SourceObjectReference. It must reach EVERY alarm subscriber so each
+        // NativeAlarmActor can atomically swap in the snapshot it just buffered.
+        // The per-source prefix match below would drop it ("".StartsWith("<src>.")
+        // is false), which would strand statically-active conditions that are only
+        // delivered in the snapshot (no later live transition) — they would buffer
+        // forever and never surface. Broadcast the sentinel to all subscribers,
+        // bypassing the source match and the condition-type filter (the sentinel
+        // carries no condition; the buffered entries were already filtered).
+        if (transition.Kind == AlarmTransitionKind.SnapshotComplete)
+        {
+            foreach (var subs in _alarmSourceSubscribers.Values)
+                foreach (var sub in subs)
+                    if (notified.Add(sub))
+                        sub.Tell(new NativeAlarmTransitionUpdate(_connectionName, transition));
+            return;
+        }
+
         foreach (var (sourceRef, subs) in _alarmSourceSubscribers)
         {
             // A subscriber bound to source S receives a transition whose source