Timer callback is not triggered when using UDP transport

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I'm using an Olimex E407 with the newly added UDP transport, and I cannot get it to work with a simple publishing example using a timer callback. If I add a breakpoint inside the callback, it's never triggered. I also tried publishing the message directly using rcl_publish, and it publishes if it's outside the timer callback. If I add a subscriber to the executor, the subscriber callback is called correctly.

According to [this issue](https://github.com/micro-ROS/micro_ros_platformio/issues/120#issuecomment-1737757093), it could be a problem with the transport (it works fine if I use DMA transport). Is this a bug or a misconfiguration on my side? What other things I could try to find what is causing the problem?

This is the full code I'm using:
```c
/* USER CODE BEGIN Header */
/**
******************************************************************************
* @file : main.c
* @brief : Main program body
******************************************************************************
* @attention
*
* Copyright (c) 2024 STMicroelectronics.
* All rights reserved.
*
* This software is licensed under terms that can be found in the LICENSE file
* in the root directory of this software component.
* If no LICENSE file comes with this software, it is provided AS-IS.
*
******************************************************************************
*/
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "cmsis_os.h"
#include "lwip.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include
#include
#include
#include
#include
#include
#include
#include
#include

/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */

#define RCCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){printf("Failed status on line %d: %d. Aborting.\n\r",__LINE__,(int)temp_rc); return true;}}
#define RCSOFTCHECK(fn) { rcl_ret_t temp_rc = fn; if((temp_rc != RCL_RET_OK)){printf("Failed status on line %d: %d. Continuing.\n\r",__LINE__,(int)temp_rc);}}

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/
UART_HandleTypeDef huart3;
DMA_HandleTypeDef hdma_usart3_rx;
DMA_HandleTypeDef hdma_usart3_tx;

/* Definitions for defaultTask */
osThreadId_t defaultTaskHandle;
const osThreadAttr_t defaultTask_attributes = {
.name = "defaultTask",
.stack_size = 3000 * 4,
.priority = (osPriority_t) osPriorityNormal,
};
/* USER CODE BEGIN PV */

rclc_support_t support;
rcl_node_t node;
rcl_timer_t timer;
rcl_allocator_t allocator;

rclc_executor_t executor;
rcl_publisher_t publisher;
std_msgs__msg__Int32 msg;

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_DMA_Init(void);
static void MX_USART3_UART_Init(void);
void StartDefaultTask(void *argument);

/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */

int _write(int file, char *ptr, int len)
{
for(int i=0 ; iInstance == TIM1) {
HAL_IncTick();
}
/* USER CODE BEGIN Callback 1 */

/* USER CODE END Callback 1 */
}

/**
* @brief This function is executed in case of error occurrence.
* @retval None
*/
void Error_Handler(void)
{
/* USER CODE BEGIN Error_Handler_Debug */
/* User can add his own implementation to report the HAL error return state */
__disable_irq();
while (1)
{
}
/* USER CODE END Error_Handler_Debug */
}

#ifdef USE_FULL_ASSERT
/**
* @brief Reports the name of the source file and the source line number
* where the assert_param error has occurred.
* @param file: pointer to the source file name
* @param line: assert_param error line source number
* @retval None
*/
void assert_failed(uint8_t *file, uint32_t line)
{
/* USER CODE BEGIN 6 */
/* User can add his own implementation to report the file name and line number,
ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
/* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

```

Thanks!

Start in the supplied main.c, especially StartDefaultTask, timer_callback, and the custom transport functions. Reproduce the UDP setup with the timer and executor, then compare its behavior with the commented DMA transport path. Done means the cause is identified and the timer callback reliably runs for the UDP configuration, with the result verified using the publishing example.

Written by the indexing model from the issue text.