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/IGalaxyAlarmFeed.cs

@@ -0,0 +1,29 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Runtime;
+
+/// <summary>
+///     Driver-side seam for the gateway's session-less alarm feed. Production wraps
+///     <c>MxGatewayClient.StreamAlarmsAsync</c> (<see cref="GatewayGalaxyAlarmFeed"/>);
+///     tests substitute a fake to drive synthetic <see cref="GalaxyAlarmTransition"/>
+///     events through <see cref="GalaxyDriver"/>'s <c>IAlarmSource</c> bridge without a
+///     running gateway.
+/// </summary>
+/// <remarks>
+///     The feed is independent of any worker session — the updated gateway serves
+///     alarms from an always-on central monitor, so the feed survives subscription
+///     churn and reconnects its own stream on transient transport failures.
+/// </remarks>
+internal interface IGalaxyAlarmFeed : IAsyncDisposable
+{
+    /// <summary>
+    ///     Fires for every alarm transition the gateway feed delivers — both the
+    ///     entries of the initial active-alarm snapshot and every subsequent live
+    ///     raise / acknowledge / clear. The OPC UA severity bucket is already mapped.
+    /// </summary>
+    event EventHandler<GalaxyAlarmTransition>? OnAlarmTransition;
+
+    /// <summary>
+    ///     Start consuming the alarm feed on a background task. Idempotent — second
+    ///     calls are no-ops while the loop is running.
+    /// </summary>
+    void Start();
+}