fix: recognize valid PMID and legacy arXiv identifiers

[njzjz-bot]

Summary

• accept historical 1–8 digit PMIDs
• support legacy arXiv identifiers and version suffixes
• require full identifier matches so trailing junk is rejected
• add regression tests for valid and malformed identifier forms

Root cause

Identifier detection used an exact 8-digit PMID regex, only supported the modern arXiv format, and called match() rather than fullmatch().

Validation

Added focused unit coverage in tests/test_identifier.py. Full repository CI should run on this PR.

Fixes #112

Agent: ChatGPT
Model: GPT-5.6 Sol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (079b00e) to head (fe18b04).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #118   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files          15       15           
  Lines         949      949           
=======================================
  Hits          896      896           
  Misses         53       53           

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