Wrong Oriented Unit Cell when Calling SlabGenerator.get_slab

I will put in a PR for this. ### Python version Python 3.11.0 ### Pymatgen version Version: 2025.10.7 (current master branch) ### Operating system version _No response_ ### Current behavior For MgO(111): a and b differ for the orientated unit cell and slab. The number of atoms per layer in the OUC is 4 whilst for the slab it is 1. <img width="897" height="498" alt="Image" src="https://github.com/user-attachments/assets/bf66fe16-1b76-4234-9724-934683de226f" /> This is expected from looking at the logic here, but I think it isn't what we want. https://github.com/materialsproject/pymatgen/blob/cc57da9bb90cf1e46ff72ca8fe8642514692cb8f/src/pymatgen/core/surface.py#L1179 ### Expected Behavior I believe the a and b lattice vectors of the OUC should be identical to those of the slab, with c in the same direction as the c vector of the slab. I might be mistaken here. https://github.com/materialsproject/pymatgen/blob/cc57da9bb90cf1e46ff72ca8fe8642514692cb8f/src/pymatgen/core/surface.py#L1179 ### Minimal example ```Python from pymatgen.core import Lattice, Structure from pymatgen.core.surface import SlabGenerator # Rocksalt MgO conventional cell bulk = Structure.from_spacegroup( "Fm-3m", Lattice.cubic(4.194), ["Mg", "O"], [[0, 0, 0], [0.5, 0.5, 0.5]], ) gen = SlabGenerator( initial_structure=bulk, miller_index=(1, 1, 1), min_slab_size=10, min_vacuum_size=20, primitive=True, lll_reduce=False, center_slab=False, max_normal_search=None, ) slab = gen.get_slabs()[0] sl = slab.lattice ol = slab.oriented_unit_cell.lattice print("slab a,b,gamma:", sl.a, sl.b, sl.gamma, sl.alpha, sl.beta) print("ouc a,b,gamma:", ol.a, ol.b, ol.gamma, ol.alpha, ol.beta) print("a/b match:", sl.a == ol.a and sl.b == ol.b) ``` ### Relevant files to reproduce this bug _No response_