Extend unit tests for get_primitive_cell() (#485)

* Extend unit tests for get_primitive_cell()

Added test cases to cover:
- Periodic boundary condition validation: ensuring ValueError is raised if structure is not periodic.
- Custom arrays warning: ensuring a warning is logged when custom arrays are present in the source structure.

Co-authored-by: jan-janssen <3854739+jan-janssen@users.noreply.github.com>

* fixes

* another fix

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: jan-janssen <3854739+jan-janssen@users.noreply.github.com>

Author: jan-janssen

tests/test_analyse_symmetry.py

@@ -3,6 +3,7 @@
 # Distributed under the terms of "New BSD License", see the LICENSE file.
 
 import unittest
+import warnings
 
 import numpy as np
 from ase.atoms import Atoms
@@ -425,6 +426,28 @@ def test_arg_equivalent_vectors(self):
         dx_round = np.round(np.absolute(dx), decimals=3)
         self.assertEqual(len(np.unique(dx_round + arg_v)), len(np.unique(arg_v)))
 
+    def test_get_primitive_cell_pbc_error(self):
+        structure = bulk("Al", cubic=True)
+        structure.pbc = [True, True, False]
+        sym = stk.analyse.get_symmetry(structure=structure)
+        with self.assertRaisesRegex(ValueError, "Can only symmetrize periodic structures."):
+            sym.get_primitive_cell()
+
+    def test_get_primitive_cell_arrays_warning(self):
+        structure = bulk("Al", cubic=True)
+        structure.set_array("test_array", np.zeros(len(structure)))
+        sym = stk.analyse.get_symmetry(structure=structure)
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter("always")
+            sym.get_primitive_cell()
+        self.assertTrue(
+            any(
+                "Custom arrays {'test_array'} do not carry over to new structure!"
+                in str(warning.message)
+                for warning in w
+            )
+        )
+
     def test_error(self):
         """spglib errors should be wrapped in a SymmetryError."""