POST /execute/${workflowId} should not have a string as a body

export async function executeWorkflow(workflowId: string) {
  return client.post<ExecuteWorkflowResponse>(
    `/execute/${workflowId}`,
    workflowId,  # <-- wrong
  );
}

So we give a string as body but the body is optionally a dictionary with execute_arguments and worker_options as possible keys

I discovered this issue when testing the conversion to fastAPI (I guess flask is more forgiving)