Fix OT unoccupied eigenvalue bandgaps

[cpignedoli]

Summary

This fixes OT HOMO-LUMO bandgap parsing for CP2K outputs where occupied and unoccupied eigenvalues are printed in separate blocks.

The parser now:

• parses occupied eigenvalues into the existing eigen_spin*_au keys;
• parses OT unoccupied eigenvalues into new unoccupied_eigen_spin*_au keys;
• parses CP2K printed HOMO - LUMO gap [eV] values into printed_bandgap_spin*_ev;
• stores bandgap_spin*_au from CP2K's printed gap when available;
• computes electron-count frontier quantities separately as eigenvalue_homo_spin*_au, eigenvalue_lumo_spin*_au, and eigenvalue_bandgap_spin*_au when both frontier eigenvalues are available;
• falls back to the electron-count gap for bandgap_spin*_au only if CP2K did not print a gap;
• leaves bandgap_spin*_au absent if no real LUMO is available;
• emits a non-fatal warning if the electron-count HOMO/LUMO gap disagrees with CP2K's printed gap.

Root Cause

After #184, eigen_spin*_au intentionally excluded the Lowest eigenvalues of the unoccupied subspace block. However, Cp2kAdvancedParser still assumed the LUMO was present in eigen_spin*_au, so for OT outputs it could silently report an occupied-occupied spacing as bandgap_spin*_au.

CI Maintenance Included

Small CI/doc maintenance changes are included because the existing checks were failing before reaching the relevant parser tests:

• updated .github/workflows/ci.yml from actions/cache@v2 to actions/cache@v4, since GitHub now hard-fails workflows using cache v2;
• added three exact AiiDA class references to docs/source/conf.py::nitpick_ignore, because the docs job runs Sphinx with -nW and was failing on unresolved external AiiDA references unrelated to this parser change;
• pinned the pyupgrade pre-commit hook runtime to avoid a pre-commit.ci crash under Python 3.14. The hook still uses --py37-plus, so this does not change the package compatibility target.

Tests

Added regression coverage using:

• the existing OT_v9.1.out fixture;
• a UKS OT fixture based on a real CP2K output, with synthetic non-numeric warning/noise lines inserted into the occupied and unoccupied eigenvalue blocks.

Local validation performed in this environment:

• direct parser check on OT_v9.1.out: stored printed gap 0.009679 eV;
• direct parser check on OT_UKS_with_warnings.out: stored printed gaps 2.811670 eV and 2.798202 eV;
• direct parser check on a real CP2K output from AiiDA calcjob 5897: stored printed gaps 2.811670 eV and 2.798202 eV;
• python -m compileall /home/jovyan/opt/aiida-cp2k/aiida_cp2k;
• pre-commit run --all-files.

Full pytest could not be run in this container because the repository conftest.py loads AiiDA PostgreSQL test fixtures and the environment is missing the locate command required by pgtest.