fix(tests,cli): implement IOpcUaAddressSpaceSink.EnsureVariable in test fakes; fix CLI CS1587

Resolves the 12 reported build errors (7 CS0535 sink fakes + 5 CLI CS1587).
Runtime.Tests green (74). NOTE: OpcUaServer.Tests still has pre-existing CS7036
errors from the in-progress Galaxy-tag workstream (Phase7Plan/Phase7CompositionResult
new required params) — separate, test-only, not addressed here.

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierHierarchyTests.cs

@@ -149,6 +149,12 @@ public sealed class Phase7ApplierHierarchyTests : IDisposable
         /// <param name="displayName">The display name of the folder.</param>
         public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
             => _calls.Enqueue((folderNodeId, parentNodeId, displayName));
+        /// <summary>Ensures a variable exists (stub implementation for testing).</summary>
+        /// <param name="variableNodeId">The node ID of the variable.</param>
+        /// <param name="parentFolderNodeId">The node ID of the parent folder, or null for root.</param>
+        /// <param name="displayName">The display name of the variable.</param>
+        /// <param name="dataType">The OPC UA built-in type name.</param>
+        public void EnsureVariable(string variableNodeId, string? parentFolderNodeId, string displayName, string dataType) { }
         /// <summary>Rebuilds the address space (stub implementation for testing).</summary>
         public void RebuildAddressSpace() { }
     }