fix(scene): route root path to editor root in create flow.

[qiuguohua]

Summary

In prefab edit mode, right-clicking with no selection (Pink sends path: '/' as parent) used to create the new node as a sibling of the prefab root, which the single-root constraint then discarded — the node "disappeared right after creation".

NodeMgr.getNodeByPath('/') returns cc.director.getScene(), which in prefab mode is the virtual scene hosting the prefab, not the prefab root. _getOrCreateNodeByPath and _getCreatePathPreflight accepted that as a truthy parent, bypassing the caller's parent = currentScene fallback (= Service.Editor.getRootNode() = the prefab root).

The component.ts add-component path already handles this with a resolveNodeByPath helper (#872). This aligns the node-create flow with the same convention.

Changes

• _getOrCreateNodeByPath (node.ts): widen the early-return guard from !path to !path || isRootNodePath(path), so root-like inputs (/, //, ///) fall through to parent = currentScene.
• _getCreatePathPreflight (node.ts): same guard on the NodeMgr.getNodeByPath short-circuit; the existing empty-pathParts fallback already produces parent: currentScene, now unmasked for /.

Tests

New file src/core/scene/test/service-core/node-create-root-path-prefab.test.ts (4 cases):

1. path: '/' in prefab mode: new empty node's setParent is called with the prefab root, not the virtual scene.
2. path: '//' normalizes to the same behaviour.
3. preflightCreate({ path: '/' }) in prefab mode plans uiTransformPath: '/PrefabRoot' when the prefab root has a UITransform — i.e. preflight sees the prefab root as parent, not the virtual scene.
4. Non-root paths (e.g. /ExistingChild) still resolve via NodeMgr.getNodeByPath unchanged.

Verified the tests catch the regression: reverting either isRootNodePath guard in node.ts makes cases 1 and 2 fail with capturedResultNode.parent equal to the virtual scene instead of the prefab root.

npx tsc --noEmit clean. jest src/core/scene/test/service-core/node-create-* — 18/18 pass (14 existing + 4 new).

QA notes

Reproduce on any prefab:

1. Open a prefab in Pink.
2. Deselect everything in the hierarchy.
3. Right-click on empty hierarchy area → create an empty node (or drag a prefab in).

Before this PR: the newly created node flashes and is gone. After: it appears as a direct child of the prefab root.

[qiuguohua]

https://zentao.sud.center/index.php?m=bug&f=view&bugID=960