refactor(opcua): FB-7 review nits — model-event SourceNode=Server, dataType guard, shape tests

Code-review follow-ups on the FB-7 surgical shape-write commit:
- GeneralModelChangeEvent now sets SourceNode=Server + SourceName (Part 3 §8.7.4)
  so clients filtering events by SourceNode match it (report still uses source:null).
- UpdateTagAttributes adds an explicit dataType null/empty guard (widened surface).
- Tighten the ArrayLengthDiffers doc comment.
- Add array→scalar transition test + null-arrayLength zero-default test (coverage
  symmetry). 275/275 OpcUaServer.Tests green.

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs

@@ -1417,6 +1417,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     public bool UpdateTagAttributes(string variableNodeId, bool writable, string? historianTagname, string dataType, bool isArray, uint? arrayLength)
     {
         ArgumentException.ThrowIfNullOrEmpty(variableNodeId);
+        ArgumentException.ThrowIfNullOrEmpty(dataType); // widened surface ⇒ explicit contract (unknown names still map to BaseDataType)
         BaseDataVariableState? shapeChangedNode = null;
         lock (Lock)
         {
@@ -1457,13 +1458,14 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         return true;
     }
 
-    /// <summary>True when the node's current 1-D array length differs from the requested one (both must be
-    /// arrays for the comparison to matter — a scalar↔array transition is already caught by the ValueRank
-    /// check in <see cref="UpdateTagAttributes"/>, so this only fires for an array-to-array length edit).</summary>
+    /// <summary>True when an array target's requested 1-D length differs from the node's current one. This is
+    /// the array-to-array length-edit case: the caller's <c>ValueRank != newValueRank</c> check already catches
+    /// any scalar↔array transition, so this runs to decide whether an already-array node's dimension changed
+    /// (and treats absent/empty current dimensions as "differs", forcing the in-place update).</summary>
     private static bool ArrayLengthDiffers(BaseDataVariableState v, bool isArray, uint? arrayLength)
     {
         if (!isArray) return false;                       // scalar target ⇒ ValueRank check owns the diff
-        if (v.ArrayDimensions is not { Count: > 0 }) return true; // was scalar/empty ⇒ shape differs
+        if (v.ArrayDimensions is not { Count: > 0 }) return true; // no current dimension ⇒ shape differs
         return v.ArrayDimensions[0] != (arrayLength ?? 0u);
     }
 
@@ -1483,6 +1485,11 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
             source: null,
             severity: EventSeverity.Medium,
             message: new LocalizedText($"Node {variable.NodeId} definition changed (DataType/ValueRank)"));
+        // Part 3 §8.7.4: a GeneralModelChangeEvent is emitted by the Server object — set SourceNode/SourceName
+        // to Server explicitly (we report with source:null since this manager has no Server NodeState handle),
+        // so conformant clients that filter events by SourceNode still match this one.
+        e.SetChildValue(SystemContext, BrowseNames.SourceNode, ObjectIds.Server, false);
+        e.SetChildValue(SystemContext, BrowseNames.SourceName, "Server", false);
         var change = new ModelChangeStructureDataType
         {
             Affected = variable.NodeId,