feat(driver-galaxy): consume the gateway's session-less alarm model

The mxaccessgw updated alarms to a session-less central monitor:
AcknowledgeAlarm dropped SessionId and alarm transitions now come from
the session-less StreamAlarms feed instead of the per-session worker
StreamEvents stream. The GalaxyDriver no longer compiled against the
updated client.

- GatewayGalaxyAlarmAcknowledger: session-less rewrite — no GalaxyMxSession;
  outcome read from ProtocolStatus (throw) and Hresult (warn).
- New IGalaxyAlarmFeed seam + GatewayGalaxyAlarmFeed: background consumer
  of StreamAlarms that decodes the active-alarm snapshot plus live
  transitions into GalaxyAlarmTransition and reopens the stream on
  transport faults.
- EventPump: drop the dead per-session OnAlarmTransition path; the
  per-session stream no longer carries alarms.
- GalaxyDriver: bridge the feed onto IAlarmSource.OnAlarmEvent; the feed
  starts on SubscribeAlarmsAsync, independent of data subscriptions.
- Tests: replace EventPumpAlarmTests with GatewayGalaxyAlarmFeedTests;
  move the driver alarm-source tests onto the IGalaxyAlarmFeed seam.

Browse needed no change — GatewayGalaxyHierarchySource consumes the
unchanged DiscoverHierarchy contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Runtime/EventPump.cs

@@ -45,12 +45,6 @@ internal sealed class EventPump : IAsyncDisposable
     private static readonly Counter<long> EventsDropped =
         Meter.CreateCounter<long>("galaxy.events.dropped", unit: "{event}",
             description: "MxEvents dropped because the bounded channel was full (newest-dropped).");
-    private static readonly Counter<long> AlarmTransitionsReceived =
-        Meter.CreateCounter<long>("galaxy.alarm_transitions.received", unit: "{event}",
-            description: "OnAlarmTransition events decoded and forwarded to driver-level handlers.");
-    private static readonly Counter<long> AlarmTransitionsDecodingFailures =
-        Meter.CreateCounter<long>("galaxy.alarm_transitions.decoding_failures", unit: "{event}",
-            description: "OnAlarmTransition events that arrived without a populated body or with an unspecified transition kind.");
 
     private readonly IGalaxySubscriber _subscriber;
     private readonly SubscriptionRegistry _registry;
@@ -66,15 +60,6 @@ internal sealed class EventPump : IAsyncDisposable
 
     public event EventHandler<DataChangeEventArgs>? OnDataChange;
 
-    /// <summary>
-    ///     Fires for every <see cref="MxEventFamily.OnAlarmTransition"/> event the
-    ///     gateway forwards. Decoded into a <see cref="GalaxyAlarmTransition"/> with
-    ///     the OPC UA severity bucket already mapped via
-    ///     <see cref="MxAccessSeverityMapper"/>. The driver wraps this onto
-    ///     <c>IAlarmSource.OnAlarmEvent</c> in PR B.2.
-    /// </summary>
-    internal event EventHandler<GalaxyAlarmTransition>? OnAlarmTransition;
-
     public EventPump(
         IGalaxySubscriber subscriber,
         SubscriptionRegistry registry,
@@ -179,13 +164,12 @@ internal sealed class EventPump : IAsyncDisposable
             case MxEventFamily.OnDataChange:
                 DispatchDataChange(ev);
                 break;
-            case MxEventFamily.OnAlarmTransition:
-                DispatchAlarmTransition(ev);
-                break;
             default:
-                // OnWriteComplete / OperationComplete / OnBufferedDataChange are filtered
-                // out — write callers get their reply via the InvokeAsync round-trip, not
-                // via the event stream.
+                // OnAlarmTransition is no longer carried on the per-session event stream
+                // — alarms come from the gateway's session-less StreamAlarms feed
+                // (GatewayGalaxyAlarmFeed). OnWriteComplete / OperationComplete /
+                // OnBufferedDataChange are filtered out: write callers get their reply
+                // via the InvokeAsync round-trip, not via the event stream.
                 return;
         }
     }
@@ -212,73 +196,6 @@ internal sealed class EventPump : IAsyncDisposable
         }
     }
 
-    private void DispatchAlarmTransition(MxEvent ev)
-    {
-        // Body absent (e.g. malformed gateway event or worker version skew) — count and
-        // drop. The Part 9 sub-attribute fallback path keeps an alarm functional even
-        // when the rich payload disappears.
-        if (ev.OnAlarmTransition is not { } body)
-        {
-            AlarmTransitionsDecodingFailures.Add(1, _clientTag);
-            _logger.LogDebug(
-                "Galaxy OnAlarmTransition event arrived without a populated body (sequence={Sequence}); ignoring.",
-                ev.WorkerSequence);
-            return;
-        }
-        if (body.TransitionKind == AlarmTransitionKind.Unspecified)
-        {
-            AlarmTransitionsDecodingFailures.Add(1, _clientTag);
-            _logger.LogDebug(
-                "Galaxy OnAlarmTransition for {AlarmRef} has unspecified transition kind; ignoring.",
-                body.AlarmFullReference);
-            return;
-        }
-
-        var (bucket, opcUaSeverity) = MxAccessSeverityMapper.Map(body.Severity);
-        var transitionTimestamp = body.TransitionTimestamp is { } tts
-            ? tts.ToDateTime()
-            : DateTime.UtcNow;
-        DateTime? originalRaiseTimestamp = body.OriginalRaiseTimestamp is { } orts
-            ? orts.ToDateTime()
-            : null;
-
-        var transition = new GalaxyAlarmTransition(
-            AlarmFullReference: body.AlarmFullReference,
-            SourceObjectReference: body.SourceObjectReference,
-            AlarmTypeName: body.AlarmTypeName,
-            TransitionKind: MapTransitionKind(body.TransitionKind),
-            SeverityBucket: bucket,
-            OpcUaSeverity: opcUaSeverity,
-            RawMxAccessSeverity: body.Severity,
-            OriginalRaiseTimestampUtc: originalRaiseTimestamp,
-            TransitionTimestampUtc: transitionTimestamp,
-            OperatorUser: body.OperatorUser,
-            OperatorComment: body.OperatorComment,
-            Category: body.Category,
-            Description: body.Description);
-
-        AlarmTransitionsReceived.Add(1, _clientTag);
-        try
-        {
-            OnAlarmTransition?.Invoke(this, transition);
-        }
-        catch (Exception ex)
-        {
-            _logger.LogWarning(ex,
-                "Galaxy OnAlarmTransition handler threw for {AlarmRef} — continuing.",
-                transition.AlarmFullReference);
-        }
-    }
-
-    private static GalaxyAlarmTransitionKind MapTransitionKind(AlarmTransitionKind kind) => kind switch
-    {
-        AlarmTransitionKind.Raise => GalaxyAlarmTransitionKind.Raise,
-        AlarmTransitionKind.Acknowledge => GalaxyAlarmTransitionKind.Acknowledge,
-        AlarmTransitionKind.Clear => GalaxyAlarmTransitionKind.Clear,
-        AlarmTransitionKind.Retrigger => GalaxyAlarmTransitionKind.Retrigger,
-        _ => GalaxyAlarmTransitionKind.Unspecified,
-    };
-
     private DataValueSnapshot ToSnapshot(MxEvent ev)
     {
         var value = MxValueDecoder.Decode(ev.Value);