Fix skill script test for default VASP potcar_prefix.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Zikkying

tests/test_skill_scripts.py

@@ -171,7 +171,7 @@ def test_build_interaction_all_backends(self):
             "vasp", incar="CUSTOM", potcars={"Al": "Al"}
         )
         self.assertEqual(vasp["incar"], "CUSTOM")
-        self.assertEqual(vasp["potcar_prefix"], ".")
+        self.assertEqual(vasp["potcar_prefix"], "vasp_potcar")
         with self.assertRaisesRegex(ValueError, "Unknown backend"):
             self.gen.build_interaction("unknown")