Expand unit coverage for untested output and volumetric parser branches

[Copilot]

This PR increases unit-test line coverage by targeting previously unexecuted control-flow paths in VASP output parsing. It adds focused branch tests where coverage was lowest, especially around fallback/error handling and structure-index remapping.

• Output parser branch coverage (tests/unit/vasp/test_output_branches.py)

  • Adds tests for parse_vasp_output paths that were not exercised:
    • VaspCollectError re-raise behavior
    • CONTCAR -> POSCAR structure fallback
    • explicit sorted_indices handling
  • Adds tests for get_final_structure_from_file branches:
    • vasp_sorter(structure) path when indices are omitted
    • non-ase.Atoms structure path via get_parent_symbols

• Volumetric parser branch coverage (tests/unit/vasp/test_volumetric_data_branches.py)

  • Adds tests for VaspVolumetricData.from_file fallback behavior:
    • _read_vol_data failure -> _read_vol_data_old fallback
    • hard failure when both parsers fail
  • Adds tests for warning/empty/corrupt paths:
    • empty file handling in _read_vol_data_old
    • missing-grid/corrupt parsing in _read_vol_data
  • Adds tests for POTCAR species fallback and to_dict() inclusion of diff data

with patch(
    "vaspparser.vasp.output.get_final_structure_from_file",
    side_effect=[OSError("contcar fail"), structure, IOError("ignore")],
):
    result = parse_vasp_output(
        working_directory=tmpdir,
        sorted_indices=[1, 0],
        output_parser_class=_ParserCollects,
    )

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.91%. Comparing base (593882f) to head (fcf07ce).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
+ Coverage   91.89%   92.91%   +1.01%     
==========================================
  Files          14       14              
  Lines        2753     2753              
==========================================
+ Hits         2530     2558      +28     
+ Misses        223      195      -28     

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