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.

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

@@ -112,6 +112,54 @@ public sealed class NodeManagerSurgicalShapeUpdateTests : IDisposable
         await host.DisposeAsync();
     }
 
+    /// <summary>The symmetric array → scalar flip: ValueRank drops back to Scalar, ArrayDimensions clears to
+    /// null, and the (now wrong-shaped) array value is reset.</summary>
+    [Fact]
+    public async Task Array_to_scalar_flip_clears_dimensions_and_resets_value()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+
+        nm.EnsureVariable("eq-1/buf", parentFolderNodeId: null, displayName: "Buf", dataType: "Int16",
+            writable: false, historianTagname: null, isArray: true, arrayLength: 4u);
+        nm.WriteValue("eq-1/buf", new short[] { 1, 2, 3, 4 }, OpcUaQuality.Good, DateTime.UtcNow);
+        var node = nm.TryGetVariable("eq-1/buf")!;
+        node.ValueRank.ShouldBe(ValueRanks.OneDimension); // arrange guard
+
+        var applied = nm.UpdateTagAttributes("eq-1/buf", writable: false, historianTagname: null,
+            dataType: "Int16", isArray: false, arrayLength: null);
+
+        applied.ShouldBeTrue();
+        node.ValueRank.ShouldBe(ValueRanks.Scalar);
+        node.ArrayDimensions.ShouldBeNull();
+        node.Value.ShouldBeNull();
+        node.StatusCode.ShouldBe((StatusCode)StatusCodes.BadWaitingForInitialData);
+
+        await host.DisposeAsync();
+    }
+
+    /// <summary>A scalar → array flip with a null arrayLength defaults ArrayDimensions to [0] — mirroring
+    /// <see cref="OtOpcUaNodeManager.EnsureVariable"/>'s fresh-node default for an unspecified length.</summary>
+    [Fact]
+    public async Task Array_flip_with_null_length_defaults_dimension_to_zero()
+    {
+        var (host, server) = await BootAsync();
+        var nm = server.NodeManager!;
+
+        nm.EnsureVariable("eq-1/buf", parentFolderNodeId: null, displayName: "Buf", dataType: "Int16", writable: false);
+        var node = nm.TryGetVariable("eq-1/buf")!;
+
+        var applied = nm.UpdateTagAttributes("eq-1/buf", writable: false, historianTagname: null,
+            dataType: "Int16", isArray: true, arrayLength: null);
+
+        applied.ShouldBeTrue();
+        node.ValueRank.ShouldBe(ValueRanks.OneDimension);
+        node.ArrayDimensions.ShouldNotBeNull();
+        node.ArrayDimensions[0].ShouldBe(0u); // null length ⇒ [0], same as EnsureVariable
+
+        await host.DisposeAsync();
+    }
+
     // ───────────────────────────── Backward compatibility (shape unchanged) ─────────────────────────────
 
     /// <summary>A Writable-only change (DataType + array-ness identical to the live node) must NOT reset the