Allow different transaction isolation levels in db and some misc changes

src/api/database/mysql_connector.py

@@ -59,7 +59,7 @@ class _MySqlDbConnection(PythonDbConnection[MySQLConnection, MySQLCursor]):
         self._closed = True
         self._py_connection.close()
     
-    def get_transaction_begin_statements(self, writable: bool, isolation_level: TransactionIsolationLevel) -> PreparedStatement or str:
+    def get_transaction_begin_statements(self, writable: bool, isolation_level: TransactionIsolationLevel) -> list[PreparedStatement or str]:
         if writable:
             return [
                 f"SET SESSION TRANSACTION ISOLATION LEVEL {isolation_level.value}",