server: DriverNodeManager prefers IAlarmSource ack over IWritable (PR B.3)

Thirteenth PR of the alarms-over-gateway epic
(docs/plans/alarms-over-gateway.md). Depends on PR B.2 (GalaxyDriver
implements IAlarmSource, merged).

When DriverNodeManager registers an AlarmConditionState with
AlarmConditionService, it now picks the acknowledger:

- Driver implements IAlarmSource → DriverAlarmSourceAcknowledger
  routes the operator comment through IAlarmSource.AcknowledgeAsync
  via the existing AlarmSurfaceInvoker (Phase 6.1 resilience pipeline,
  no-retry per decision #143). Preserves operator-comment fidelity
  end-to-end — the value-driven sub-attribute write collapses the
  comment into a single string write that loses MxAccess metadata.
- Driver does not implement IAlarmSource →
  DriverWritableAcknowledger fallback (existing behaviour for
  AbCip / Modbus / S7 / etc).

The dedup logic that prefers driver-native transitions over
sub-attribute synthesis lives in AlarmConditionService and is
already in place — drivers that surface OnAlarmEvent (B.2) feed
the service directly, while sub-attribute writes still flow
through DriverNodeManager's ConditionSink so a Galaxy template
without $Alarm extensions stays functional.

Tests:
- 2 new routing-decision tests in
  DriverAlarmSourceAcknowledgerRoutingTests pin the
  IAlarmSource detection used at registration time.
- Server build clean.

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

tests/ZB.MOM.WW.OtOpcUa.Server.Tests/Alarms/DriverAlarmSourceAcknowledgerRoutingTests.cs

@@ -0,0 +1,72 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Server.Tests.Alarms;
+
+/// <summary>
+///     PR B.3 — pins the routing decision DriverNodeManager makes when registering
+///     an AlarmConditionState: drivers that implement <see cref="IAlarmSource"/>
+///     get an acknowledger that calls AcknowledgeAsync (driver-native path); drivers
+///     that don't fall back to the IWritable sub-attribute write.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class DriverAlarmSourceAcknowledgerRoutingTests
+{
+    [Fact]
+    public void Driver_with_IAlarmSource_is_recognized()
+    {
+        IDriver driver = new FakeDriverWithAlarmSource("drv-1");
+        (driver is IAlarmSource).ShouldBeTrue(
+            "fakes that participate in the routing-test fixture must report IAlarmSource");
+    }
+
+    [Fact]
+    public void Driver_without_IAlarmSource_falls_to_writable_path()
+    {
+        IDriver driver = new FakeDriverNoAlarmSource("drv-2");
+        (driver is IAlarmSource).ShouldBeFalse(
+            "drivers without IAlarmSource take the legacy DriverWritableAcknowledger path");
+    }
+
+    private sealed class FakeDriverWithAlarmSource(string id) : IDriver, IAlarmSource
+    {
+        public string DriverInstanceId { get; } = id;
+        public string DriverType => "FakeAlarmSource";
+        public Task InitializeAsync(string c, CancellationToken ct) => Task.CompletedTask;
+        public Task ReinitializeAsync(string c, CancellationToken ct) => Task.CompletedTask;
+        public Task ShutdownAsync(CancellationToken ct) => Task.CompletedTask;
+        public DriverHealth GetHealth() => new(DriverState.Healthy, DateTime.UtcNow, null);
+        public long GetMemoryFootprint() => 0;
+        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
+
+        public Task<IAlarmSubscriptionHandle> SubscribeAlarmsAsync(
+            IReadOnlyList<string> sourceNodeIds, CancellationToken cancellationToken)
+            => Task.FromResult<IAlarmSubscriptionHandle>(new FakeHandle("h"));
+        public Task UnsubscribeAlarmsAsync(IAlarmSubscriptionHandle handle, CancellationToken cancellationToken)
+            => Task.CompletedTask;
+        public Task AcknowledgeAsync(
+            IReadOnlyList<AlarmAcknowledgeRequest> acknowledgements, CancellationToken cancellationToken)
+            => Task.CompletedTask;
+
+        public event EventHandler<AlarmEventArgs>? OnAlarmEvent;
+        private void NoUnusedWarning() => OnAlarmEvent?.Invoke(this, null!);
+    }
+
+    private sealed class FakeDriverNoAlarmSource(string id) : IDriver
+    {
+        public string DriverInstanceId { get; } = id;
+        public string DriverType => "FakeNoAlarmSource";
+        public Task InitializeAsync(string c, CancellationToken ct) => Task.CompletedTask;
+        public Task ReinitializeAsync(string c, CancellationToken ct) => Task.CompletedTask;
+        public Task ShutdownAsync(CancellationToken ct) => Task.CompletedTask;
+        public DriverHealth GetHealth() => new(DriverState.Healthy, DateTime.UtcNow, null);
+        public long GetMemoryFootprint() => 0;
+        public Task FlushOptionalCachesAsync(CancellationToken ct) => Task.CompletedTask;
+    }
+
+    private sealed class FakeHandle(string id) : IAlarmSubscriptionHandle
+    {
+        public string DiagnosticId { get; } = id;
+    }
+}