Fix PDB parser for short lines

[diegozea]

Summary

• warn once when occupancy is missing using maxlog=1
• avoid state flag and strip occupancy and B-factor once
• use Unicode comparison operators
• extend short PDB sample and regression test

Testing

• julia --project -e 'push!(LOAD_PATH, "test"); using MIToSTests; MIToSTests.retest("PDB"); MIToSTests.retest("PDB")'
• julia --project -e 'using Pkg; Pkg.test(coverage=true)' (interrupted: tests hung)

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https://chatgpt.com/codex/tasks/task_b_684c33668ea4832d9a11ee331974d510