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/DeferredAddressSpaceSinkTests.cs

@@ -84,6 +84,9 @@ public sealed class DeferredAddressSpaceSinkTests
         public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
             => CallQueue.Enqueue($"EF:{folderNodeId}");
         /// <inheritdoc />
+        public void EnsureVariable(string variableNodeId, string? parentFolderNodeId, string displayName, string dataType)
+            => CallQueue.Enqueue($"EV:{variableNodeId}");
+        /// <inheritdoc />
         public void RebuildAddressSpace() => CallQueue.Enqueue("RB");
     }
 }