test: add negative cases and fix DER public-key verification

[kostiask]

Summary

This PR strengthens test coverage for the CLI command flow and fixes a bug in verify when using DER public keys.

What changed

• Added negative-path tests:
  • generate returns error for invalid base64 entropy.
  • sign returns I/O error for missing secret key/input file.
  • verify returns I/O error for missing public key/signature.
• Expanded verification matrix:
  • verify_all_algorithms_all_formats now covers both PEM and DER.
• Fixed DER verification path in verify:
  • corrected extraction/handling of public key bytes from DER SubjectPublicKeyInfo.
  • removed outdated/unused BitString import after refactor.
• Kept all tests clippy-clean under strict settings (-D warnings).