Add OPC UA array element write integration test

tests/ZB.MOM.WW.LmxOpcUa.Tests/Helpers/OpcUaTestClient.cs

@@ -114,6 +114,27 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
             return Session.ReadValue(nodeId);
         }
 
+        /// <summary>
+        /// Write a node's value, optionally using an OPC UA index range for array element writes.
+        /// Returns the server status code for the write.
+        /// </summary>
+        public StatusCode Write(NodeId nodeId, object value, string? indexRange = null)
+        {
+            var nodesToWrite = new WriteValueCollection
+            {
+                new WriteValue
+                {
+                    NodeId = nodeId,
+                    AttributeId = Attributes.Value,
+                    IndexRange = indexRange,
+                    Value = new DataValue(new Variant(value))
+                }
+            };
+
+            Session.Write(null, nodesToWrite, out var results, out _);
+            return results[0];
+        }
+
         /// <summary>
         /// Create a subscription with a monitored item on the given node.
         /// Returns the subscription and monitored item for inspection.