feat(contracts): carry operator credentials on alarm acknowledgement

Claude-Session: https://claude.ai/code/session_016J6isfcgqy4SB2RK3ZvFvT

src/ZB.MOM.WW.MxGateway.Contracts/WorkerLaunchEnvironment.cs

@@ -22,4 +22,10 @@ public static class WorkerLaunchEnvironment
 
     /// <summary>Maximum alarm records requested in one current-alarm fetch.</summary>
     public const string MaxAlarmsPerFetch = "MXGATEWAY_ALARM_MAX_ALARMS_PER_FETCH";
+
+    /// <summary>
+    ///     MXAccess client name the worker's alarm-ack writer registers with. The
+    ///     Historian records it as the acknowledgement's OperatorNode.
+    /// </summary>
+    public const string AlarmAckClientName = "MXGATEWAY_ALARM_ACK_CLIENT_NAME";
 }