test: assert resolved position depth triggers the guard

Verify that the resolved position's depth is < 1 before calling
getParentBlockInfo, ensuring we exercise the top-level branch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hedi-ghodhbane

packages/core/src/api/blockManipulation/commands/mergeBlocks/mergeBlocks.test.ts

@@ -82,6 +82,10 @@ describe("Test mergeBlocks", () => {
 
     const beforePos = getPosBeforeSelectedBlock();
     const doc = getEditor()._tiptapEditor.state.doc;
+    const $pos = doc.resolve(beforePos);
+
+    expect($pos.depth - 1).toBeLessThan(1);
+
     const result = getParentBlockInfo(doc, beforePos);
 
     expect(result).toBeUndefined();