Arduino Wire.h I2C custom transport on Pi Pico is causing a crash

- Hardware description: Pi Pico
- RTOS: I think arduino mbed
- Version or commit hash: humble

#### Steps to reproduce the issue
I'm trying to make a custom transport for I2C using arduino's Wire library. I am able to build and flash the microros_publisher example, everything works over serial and I can see the topic and node in a docker container. However, when I replace setting the serial transport with my own, I think the pico crashes since i'm getting a flashing pattern on the pico (4 long, 4 0r 5 long). I really hope I'm missing something subtle. Also I'm not sure how I am supposed to properly use `uxr_init_custom_transport`, or if I'm supposed to use it at all, because the default serial transport doesn't. Also if this isn't the correct place to post this issue, please point me in the right direction.

here's my ini

```
env:pico]
platform = https://github.com/maxgerhardt/platform-raspberrypi.git
board = pico
framework = arduino
lib_deps =
https://github.com/micro-ROS/micro_ros_platformio

upload_protocol = mbed

board_microros_distro = humble
board_microros_transport = custom
```

heres the code

https://pastebin.com/5hzPiRcD

Beginne mit der PlatformIO-ini und dem verknüpften Pastebin-Code und konzentriere dich darauf, wie der benutzerdefinierte Transport konfiguriert ist und wo `uxr_init_custom_transport` verwendet wird. Reproduziere zunächst die serielle Baseline und anschließend den I2C-Transport auf dem Pi Pico; fertig ist die Aufgabe, wenn der benutzerdefinierte Transport das Board nicht mehr zum Absturz bringt und der micro-ROS-Knoten sowie das Topic weiterhin sichtbar sind.

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