_validate_zip_entry has no ML-stack dependency; importing it via main.py
pulled in surya/torch and caused the test to be skipped in CI. Moving it
to utils.py (fastapi only) and adding fastapi to the CI lightweight install
lets test_zipslip_still_anchors_under_custom_tmpdir run on every push.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
