Fix AttributeError in VaspInput.as_dict() and add test for potcar_spe…

[cogsworth37]

…c handling

Fixes materialsproject/pymatgen#3821

closes

Summary

Major changes:

• Fix AttributeError in VaspInput.as_dict(). when VaspInput(potcar_spec=True), the potcar key has a string value that doesn't have a dict() attribute.
• Wrote unit tests to cover the scenario

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install