Guard rename_child collisions

Raise when renaming a child to an existing sibling name to prevent overwriting
entries in the children hash. This avoids orphaning nodes and keeps child
lookup consistent.

Author: evolve75

lib/tree.rb

@@ -457,6 +457,9 @@ def rename_child(old_name, new_name)
       raise ArgumentError, "Invalid child name specified: #{old_name}"\
             unless @children_hash.key?(old_name)
 
+      raise ArgumentError, "Child name already exists: #{new_name}"\
+            if @children_hash.key?(new_name)
+
       @children_hash[new_name] = @children_hash.delete(old_name)
       @children_hash[new_name].name = new_name
     end

test/test_tree.rb

@@ -1545,6 +1545,9 @@ def test_rename_child
 
       assert_raise(ArgumentError) { @root.rename_child('Not_Present_Child1', 'ALT_Child1') }
 
+      error = assert_raise(ArgumentError) { @root.rename_child('Child1', 'Child2') }
+      assert_match(/Child name already exists: Child2/, error.message)
+
       @root.rename_child('Child1', 'ALT_Child1')
       assert_equal('ALT_Child1', @child1.name, "Name should be 'ALT_Child1'")
       assert_equal(@child1, @root['ALT_Child1'], 'Should be able to access from parent using new name')