deleteExecution method
Deletes a workflow execution and all associated data (checkpoints, timers, signals).
Use this to clean up completed or failed executions.
Implementation
@override
Future<void> deleteExecution(String workflowExecutionId) async {
_executions.remove(workflowExecutionId);
_checkpoints.remove(workflowExecutionId);
_timers.removeWhere((t) => t.workflowExecutionId == workflowExecutionId);
_signals.removeWhere((s) => s.workflowExecutionId == workflowExecutionId);
}