Persist schema migrations

src/tests/test_migrations.py

@@ -78,3 +78,29 @@ def test_error_on_future_version(tmp_path: Path):
     enc_mgr.save_json_data(future)
     with pytest.raises(ValueError):
         vault.load_index()
+
+
+def test_schema_migration_persisted_once(tmp_path: Path):
+    enc_mgr, vault = setup(tmp_path)
+    legacy = {
+        "schema_version": 3,
+        "entries": {
+            "0": {
+                "label": "a",
+                "length": 8,
+                "type": "password",
+                "notes": "",
+                "custom_fields": [],
+                "origin": "",
+            }
+        },
+    }
+    enc_mgr.save_json_data(legacy)
+    data, migrated = vault.load_index(return_migration_flag=True)
+    assert migrated is True
+    assert data["schema_version"] == LATEST_VERSION
+    assert data["entries"]["0"]["tags"] == []
+
+    data_again, migrated_again = vault.load_index(return_migration_flag=True)
+    assert migrated_again is False
+    assert data_again == data