test: update AMBER helper import after format move

Authored by OpenClaw (model: custom-chat-jinzhezeng-group/gpt-5.5)

Author: njzjz-bot

tests/test_amber_md.py

@@ -9,7 +9,7 @@
 from comp_sys import CompLabeledSys, IsPBC
 from context import dpdata
 
-from dpdata.amber.md import cell_lengths_angles_to_cell
+from dpdata.formats.amber.md import cell_lengths_angles_to_cell
 
 try:
     import parmed  # noqa: F401