Verified all test helper functions, fixtures, and constants for encryption
testing are complete and functional per bead bf-1b9yf.
- Constants: EXIT_ENCRYPTED = 3 ✓
- Fixtures: 5 encrypted PDFs exist and are valid ✓
- Path functions: workspace_root, pdftract_bin, encrypted_fixture all work ✓
- Test helpers: run_pdftract_extract, assertion helpers all compile ✓
- Mock builders: dict builders for RC4/AES encryption ready ✓
- Verification tests: all pass in verify_encryption_fixtures.rs ✓
Closes bf-1b9yf
