[FIX] Handle collapsed folder drops in manager tree

[middleDuckAi]

Problem

Dropping a resource into a collapsed folder goes through the folder-drop branch in the manager tree, but that branch only appended into the target child container when the container already had rendered HTML.

Collapsed folders still have a valid child container, but it starts empty, so the move flow becomes inconsistent and the resource can appear to snap back after refresh.

What changed

• extracted the folder-drop branch into a shared manager/media/script/tree-drop-helper.js
• loaded the helper before theme modx.js in the manager frame
• updated both default and liquid manager themes to use the same folder-drop helper
• added a regression test for the empty collapsed-folder container case

Why this shape

• keeps both manager themes in sync
• fixes the empty-container path without changing before/after sibling drop behavior
• makes the fragile branch testable outside a browser-only harness

Files

• manager/media/script/tree-drop-helper.js
• manager/media/script/tests/tree-drop-helper.test.js
• manager/views/frame/1.blade.php
• manager/media/style/default/js/modx.js
• manager/media/style/liquid/js/modx.js

Verification

• node --test manager/media/script/tests/tree-drop-helper.test.js
• node --check manager/media/style/default/js/modx.js
• node --check manager/media/style/liquid/js/modx.js
• php -l manager/views/frame/1.blade.php
• local runtime smoke check on the task branch

Issue

• closes Drag&drop into a collapsed folder #2194