Add unit tests for hash_password and verify_password

## Description Add unit tests for the `hash_password` and `verify_password` functions. The tests should be added to: **`tests/unit/test_user.py`** ## Acceptance criteria - Confirm that `hash_password()` does not return the original plaintext password. - Confirm that hashing the same password twice produces different hashes. - Confirm that `verify_password()` returns `True` for the correct password. - Confirm that `verify_password()` returns `False` for an incorrect password. - Do not use the database or make HTTP requests. - All relevant tests must pass locally and in GitHub Actions. ## Contribution notes Please comment on this issue before starting work. Keep the pull request limited to this issue and follow `CONTRIBUTING.md`.