Fix structure field shadowing sort_structure in VaspInputGenerator

[ThomasWarford]

Summary

• VaspInputGenerator redeclared structure as a plain dataclass field, overriding the structure property (with its sort_structure/reduce_structure/validate_magmom setter) inherited from pymatgen's VaspInputSet. As a result, sort_structure silently never ran for any atomate2 VASP input-set generator.
• This surfaces destructively in MDSetGenerator's npt ensemble: LANGEVIN_GAMMA is sized to the number of distinct elements, while VASP counts POSCAR "types" as contiguous same-element runs. With an unsorted structure the two counts diverge and VASP aborts with Error reading item LANGEVIN_GAMMA from file INCAR.
• Fix: remove the redundant field declaration so the inherited property is no longer shadowed.
• Added regression tests (tests/vasp/test_sets.py) and a standalone repro (example.py).

Alternatively, we modify MDSetGenerator something like:

class MDSetGenerator(VaspInputGenerator):
    def get_input_set(self, structure=None, prev_dir=None, potcar_spec=False):
        if structure is not None and self.sort_structure:
            structure = structure.get_sorted_structure()
        return super().get_input_set(structure, prev_dir=prev_dir, potcar_spec=potcar_spec)

for a more targeted fix.

Additional dependencies introduced (if any)

• None.

TODO (if any)

• None.

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Issue

Closes #1529. See this issue for bug reproduction instructions.

[JaGeo]

@esoteric-ephemera do you have time to look into this? I don't have much experience with the MD workflow

[esoteric-ephemera]

remove the ai-generated comment please

[esoteric-ephemera]

Thanks @ThomasWarford. I'm not sure the analysis is quite right, since VaspInputSet itself defines structure as an init field, and then later defines it as a property with a setter. My guess is that there's some unhealthy handling/collision of fields in the upstream model that need to be changed

That being said, it is redundant to redeclare the field here so I'm fine with this change

Guessing example.py was an AI-generated MWE?