fix(GWC-33): unregister terminal worker queue gauge

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClient.cs

@@ -444,11 +444,10 @@ public sealed class WorkerClient : IWorkerClient
             {
                 _state = WorkerClientState.Closed;
             }
+
+            UnregisterEventQueueDepthSource();
         }
 
-        // Drop out of the worker queue-depth gauge before teardown: whatever this client still holds
-        // is about to be discarded, and the sum must not keep counting a client that is going away.
-        _eventQueueDepthRegistration?.Dispose();
         KillOwnedProcess("Dispose");
         _stopCts.Cancel();
         _outboundEnvelopes.Writer.TryComplete();
@@ -1092,6 +1091,7 @@ public sealed class WorkerClient : IWorkerClient
             }
 
             _state = WorkerClientState.Closed;
+            UnregisterEventQueueDepthSource();
         }
 
         _stopCts.Cancel();
@@ -1126,6 +1126,7 @@ public sealed class WorkerClient : IWorkerClient
             }
 
             _state = WorkerClientState.Faulted;
+            UnregisterEventQueueDepthSource();
         }
 
         _stopCts.Cancel();
@@ -1139,6 +1140,16 @@ public sealed class WorkerClient : IWorkerClient
         _logger.LogWarning(exception, "Worker client faulted for session {SessionId}: {Message}", SessionId, message);
     }
 
+    /// <summary>
+    ///     Removes this terminal client's abandoned backlog from the gateway-wide queue-depth
+    ///     gauge. The registration handle is idempotent, so terminal transitions and later disposal
+    ///     may all call this safely while the faulted client remains retained for diagnostics.
+    /// </summary>
+    private void UnregisterEventQueueDepthSource()
+    {
+        _eventQueueDepthRegistration?.Dispose();
+    }
+
     private void KillOwnedProcess(string reason)
     {
         WorkerProcessHandle? processHandle = _connection.ProcessHandle;