fix(dock): fix InsertChild move semantics and improve test quality

src/Editor/Ghost.Editor.Core/Controls/Internal/Docking/DockGroupNode.cs

@@ -74,7 +74,17 @@ public partial class DockGroupNode : DockNode
             int oldIndex = _children.IndexOf(node);
             if (oldIndex == index) return;
 
-            _children.Move(oldIndex, index);
+            // ObservableCollection.Move(oldIndex, newIndex) requires newIndex < Count.
+            // If index is Count, we move it to the last position (Count - 1).
+            int targetIndex = index;
+            if (targetIndex >= _children.Count)
+            {
+                targetIndex = _children.Count - 1;
+            }
+
+            if (oldIndex == targetIndex) return;
+
+            _children.Move(oldIndex, targetIndex);
             return;
         }
 

src/Editor/Ghost.Editor/View/Controls/DockLayout.cs

@@ -414,7 +414,7 @@ public sealed partial class DockLayout : Control
     /// Applies the tree mutation for a drop operation.
     /// </summary>
     /// <returns>True if the mutation was applied; otherwise, false.</returns>
-    private bool TryApplyDropMutation(DockPanelNode targetNode, DockPanelNode sourceNode, object item, DockPosition position)
+    internal bool TryApplyDropMutation(DockPanelNode targetNode, DockPanelNode sourceNode, object item, DockPosition position)
     {
         if (position == DockPosition.Center)
         {

src/Test/Ghost.UnitTest/DockingMutationTest.cs

@@ -19,11 +19,12 @@ public class DockingMutationTest
         root.AddChild(panel2);
 
         // Simulate Center Drop: panel1 -> panel2
-        panel1.Items.Remove(item);
+        bool removed = panel1.Items.Remove(item);
+        Assert.IsTrue(removed);
         panel2.Items.Add(item);
 
-        Assert.AreEqual(0, panel1.Items.Count);
-        Assert.AreEqual(1, panel2.Items.Count);
+        Assert.IsEmpty(panel1.Items);
+        Assert.HasCount(1, panel2.Items);
         Assert.AreEqual(item, panel2.Items[0]);
     }
 
@@ -39,7 +40,8 @@ public class DockingMutationTest
 
         // Simulate Vertical Split Drop (Bottom) on panel1
         // 1. Remove from source
-        panel1.Items.Remove(item);
+        bool removed = panel1.Items.Remove(item);
+        Assert.IsTrue(removed);
         
         // 2. Different orientation (Vertical), replace panel1 with new group
         root.RemoveChild(panel1);
@@ -51,9 +53,9 @@ public class DockingMutationTest
         newGroup.AddChild(newNode); // New split
         root.AddChild(newGroup);
 
-        Assert.AreEqual(1, root.Children.Count);
+        Assert.HasCount(1, root.Children);
         Assert.AreEqual(newGroup, root.Children[0]);
-        Assert.AreEqual(2, newGroup.Children.Count);
+        Assert.HasCount(2, newGroup.Children);
         Assert.AreEqual(panel1, newGroup.Children[0]);
         Assert.AreEqual(newNode, newGroup.Children[1]);
         Assert.AreEqual(item, newNode.Items[0]);
@@ -82,7 +84,7 @@ public class DockingMutationTest
             root.RemoveChild(group1);
         }
 
-        Assert.AreEqual(0, root.Children.Count);
+        Assert.IsEmpty(root.Children);
         Assert.IsNull(group1.Parent);
         Assert.IsNull(panel1.Parent);
     }
@@ -114,7 +116,7 @@ public class DockingMutationTest
             parent.InsertChild(index, onlyChild);
         }
 
-        Assert.AreEqual(1, root.Children.Count);
+        Assert.HasCount(1, root.Children);
         Assert.AreEqual(panel1, root.Children[0]);
         Assert.AreEqual(root, panel1.Parent);
     }