Add test for legacy Fernet data with V2 prefix

src/tests/test_v2_prefix_fallback.py

@@ -0,0 +1,21 @@
+import logging
+from pathlib import Path
+
+from helpers import TEST_SEED
+from utils.key_derivation import derive_index_key
+from password_manager.encryption import EncryptionManager
+
+
+def test_v2_prefix_fernet_fallback(tmp_path: Path, caplog) -> None:
+    key = derive_index_key(TEST_SEED)
+    manager = EncryptionManager(key, tmp_path)
+
+    original = b"legacy data"
+    token = manager.fernet.encrypt(original)
+    payload = b"V2:" + token
+
+    caplog.set_level(logging.WARNING, logger="password_manager.encryption")
+    decrypted = manager.decrypt_data(payload)
+
+    assert decrypted == original
+    assert "incorrect 'V2:' header" in caplog.text