fix: add validation to prevent renaming to existing directory name (qa-requested)

Fixes:
- Add duplicate directory check in VFS.Rename.cs before rename operation
- Add test to verify exception is thrown when renaming to existing name

Verified:
- Validation follows same pattern as CreateDirectory duplicate check
- Test matches pattern from existing duplicate validation tests
- Fixes acceptance criteria #5: renaming to existing name throws exception

QA Fix Session: 1

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: phmatray

src/Atypical.VirtualFileSystem.Core/SystemOperations/Commands/Rename/VFS.Rename.cs

@@ -37,6 +37,11 @@ public IVirtualFileSystem RenameDirectory(
         // update the directory node with the new path
         var oldName = directoryNode.Name;
         var newPath = new VFSDirectoryPath($"{directoryPath.Parent}/{newName}");
+
+        // Validate that the destination path doesn't already exist
+        if (Index.ContainsKey(newPath))
+            ThrowVirtualNodeAlreadyExists(newPath);
+
         var updatedDirectoryNode = directoryNode.UpdatePath(newPath);
 
         // Add the directory to its old parent directory with the new name

tests/Atypical.VirtualFileSystem.UnitTests/SystemOperations/Commands/VirtualFileSystem_MethodRenameDirectory_Tests.cs

@@ -290,4 +290,23 @@ public void RenameDirectory_handles_deeply_nested_structure()
         _vfs.Index[newSubdirAtC].Path.Value.Should().Be("vfs://a/b/renamed_c/subdir_c");
         _vfs.Index[newSubdirAtE].Path.Value.Should().Be("vfs://a/b/renamed_c/d/e/subdir_e");
     }
+
+    [Fact]
+    public void RenameDirectory_throws_exception_when_target_name_already_exists()
+    {
+        // Arrange
+        _vfs.CreateDirectory(new VFSDirectoryPath("dir1/dir2/dir3"));
+        _vfs.CreateDirectory(new VFSDirectoryPath("dir1/dir2/existing"));
+
+        // Act
+        Action action = () => _vfs.RenameDirectory(
+            new VFSDirectoryPath("dir1/dir2/dir3"),
+            "existing"
+        );
+
+        // Assert
+        action.Should()
+            .Throw<VirtualFileSystemException>()
+            .WithMessage("The node 'vfs://dir1/dir2/existing' already exists in the index.");
+    }
 }