exec abstract method
Executes all commands queued after multi().
Returns a List<dynamic> of replies for each command in the transaction.
Returns null if the transaction was aborted (e.g., due to a WATCH failure).
Throws a ValkeyServerException (e.g., EXECABORT) if the transaction
was discarded due to a command syntax error within the MULTI block.
Implementation
Future<List<dynamic>?> exec();