fix: preserve CTER and NTER residue names

[nightcityblade]

Fixes #454

Summary

• preserve explicit CTER and NTER terminal names from force field output
• keep stripping the C/N prefix for protonated residue names like CGLU and NHIS
• add a regression test covering both terminal-name preservation and prefix stripping

Testing

• python -m pytest tests/core_test.py -k "terminal_forcefield_names_preserve_termini or protonated_terminals" -q
• python -m ruff check --config ruff_essential.toml pdb2pqr/biomolecule.py tests/core_test.py

[sobolevnrm]

Hi @nightcityblade -- thanks for submitting this. Can you please fix the linting errors so the tests pass? Thanks!