fix(communication): drop IsConfiguredPlaceholder rows in StreamRelayActor before gRPC pack

Placeholder AlarmStateChanged rows are a DebugView snapshot-only concept emitted
by InstanceActor.BuildAlarmStatesSnapshot; they are never a real alarm transition.
Their timestamp may be DateTimeOffset.MinValue (the Protobuf Timestamp lower boundary),
which can throw when packed via Timestamp.FromDateTimeOffset.

Added early-return guard at the top of HandleAlarmStateChanged before any timestamp
pack or channel write. Updated the existing NativeBindingLinkage round-trip test to
use a real (non-placeholder) native alarm; added DropsAlarmStateChanged_WhenIsConfiguredPlaceholder
to assert placeholders are silently dropped (15/15 pass).

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/StreamRelayActor.cs

@@ -56,6 +56,15 @@ public class StreamRelayActor : ReceiveActor
 
     private void HandleAlarmStateChanged(AlarmStateChanged msg)
     {
+        // Placeholder rows (IsConfiguredPlaceholder) are a Debug View snapshot-only
+        // concept emitted by InstanceActor.BuildAlarmStatesSnapshot — they are never a
+        // real alarm transition and must not be relayed to the live gRPC stream (their
+        // timestamp may be DateTimeOffset.MinValue, the Protobuf Timestamp boundary).
+        if (msg.IsConfiguredPlaceholder)
+        {
+            return;
+        }
+
         var protoEvent = new SiteStreamEvent
         {
             CorrelationId = _correlationId,

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/StreamRelayActorTests.cs

@@ -116,9 +116,9 @@ public class StreamRelayActorTests : TestKit
     [Fact]
     public void RelaysAlarmStateChanged_NativeBindingLinkage_SurvivesFullRoundTrip()
     {
-        // DV-1: the native source-binding canonical name and the configured-placeholder
-        // flag must pack into AlarmStateUpdate (StreamRelayActor) and unpack back out
-        // (SiteStreamGrpcClient.ConvertToDomainEvent) — the full cross-process round trip.
+        // DV-1: the native source-binding canonical name must pack into AlarmStateUpdate
+        // (StreamRelayActor) and unpack back out (SiteStreamGrpcClient.ConvertToDomainEvent)
+        // — the full cross-process round trip for a real (non-placeholder) native alarm.
         var channel = Channel.CreateUnbounded<SiteStreamEvent>();
         var actor = Sys.ActorOf(Props.Create(() =>
             new StreamRelayActor("corr-native-binding", channel.Writer)));
@@ -130,7 +130,7 @@ public class StreamRelayActorTests : TestKit
             Kind = AlarmKind.NativeOpcUa,
             SourceReference = "Motor1.MotorAlarms.Hi",
             NativeSourceCanonicalName = "Motor1.MotorAlarms",
-            IsConfiguredPlaceholder = true,
+            IsConfiguredPlaceholder = false,
             Condition = new AlarmConditionState(
                 Active: true, Acknowledged: false, Confirmed: null,
                 Shelve: AlarmShelveState.Unshelved, Suppressed: false, Severity: 900)
@@ -150,13 +150,46 @@ public class StreamRelayActorTests : TestKit
 
         // Packed onto the wire frame by StreamRelayActor.
         Assert.Equal("Motor1.MotorAlarms", protoEvent.AlarmChanged.NativeSourceCanonicalName);
-        Assert.True(protoEvent.AlarmChanged.IsConfiguredPlaceholder);
+        Assert.False(protoEvent.AlarmChanged.IsConfiguredPlaceholder);
 
         // Unpacked back into the domain record by SiteStreamGrpcClient.
         var roundTripped = Assert.IsType<AlarmStateChanged>(
             SiteStreamGrpcClient.ConvertToDomainEvent(protoEvent));
         Assert.Equal("Motor1.MotorAlarms", roundTripped.NativeSourceCanonicalName);
-        Assert.True(roundTripped.IsConfiguredPlaceholder);
+        Assert.False(roundTripped.IsConfiguredPlaceholder);
+    }
+
+    [Fact]
+    public void DropsAlarmStateChanged_WhenIsConfiguredPlaceholder()
+    {
+        // Placeholder rows are a Debug View snapshot-only concept emitted by
+        // InstanceActor.BuildAlarmStatesSnapshot. They are never a real alarm
+        // transition; their timestamp may be DateTimeOffset.MinValue (the Protobuf
+        // Timestamp boundary). The relay must drop them without writing to the channel.
+        var channel = Channel.CreateUnbounded<SiteStreamEvent>();
+        var actor = Sys.ActorOf(Props.Create(() =>
+            new StreamRelayActor("corr-placeholder-drop", channel.Writer)));
+
+        var domainEvent = new AlarmStateChanged(
+            "Site1.Tank01", "Tank1.Levels.Hi", AlarmState.Normal, 500,
+            DateTimeOffset.MinValue)
+        {
+            Kind = AlarmKind.NativeOpcUa,
+            NativeSourceCanonicalName = "Tank1.Levels",
+            IsConfiguredPlaceholder = true,
+            Condition = new AlarmConditionState(
+                Active: false, Acknowledged: false, Confirmed: null,
+                Shelve: AlarmShelveState.Unshelved, Suppressed: false, Severity: 0)
+        };
+
+        actor.Tell(domainEvent);
+
+        // Allow time for the actor to process the message.
+        Thread.Sleep(500);
+
+        // Nothing must have been written — placeholder is silently dropped.
+        Assert.False(channel.Reader.TryRead(out _),
+            "Placeholder AlarmStateChanged must not be relayed to the gRPC stream");
     }
 
     [Fact]