Add test for parent seed migration

src/tests/test_seed_migration.py

@@ -0,0 +1,31 @@
+import sys
+from pathlib import Path
+from cryptography.fernet import Fernet
+
+from helpers import TEST_PASSWORD, TEST_SEED
+from utils.key_derivation import derive_key_from_password
+
+sys.path.append(str(Path(__file__).resolve().parents[1]))
+
+from password_manager.encryption import EncryptionManager
+
+
+def test_parent_seed_migrates_from_fernet(tmp_path: Path) -> None:
+    key = derive_key_from_password(TEST_PASSWORD)
+    fernet = Fernet(key)
+    encrypted = fernet.encrypt(TEST_SEED.encode())
+    legacy_file = tmp_path / "parent_seed.enc"
+    legacy_file.write_bytes(encrypted)
+
+    manager = EncryptionManager(key, tmp_path)
+    decrypted = manager.decrypt_parent_seed()
+
+    assert decrypted == TEST_SEED
+
+    new_file = tmp_path / "parent_seed.enc"
+    legacy_backup = tmp_path / "parent_seed.enc.fernet"
+
+    assert new_file.exists()
+    assert legacy_backup.exists()
+    assert new_file.read_bytes() != encrypted
+    assert legacy_backup.read_bytes() == encrypted