fix: replace string-prefix check in safe_extractall with commonpath

`startswith` is the wrong primitive for "is target inside directory base":
once a trailing separator drops anywhere along the way, sibling directories
whose names extend base match. We could spot-fix by re-appending the
separator before the check, but `commonpath` makes the directory-boundary
intent explicit and removes the failure mode entirely.

Fixes GHSA-6cmm-8875-5pcw.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>