[LVGL+flow] system structs not listed in input and output type dropdown.

[fietser28]

Describe the bug
Available SYSTEM structs are not listed in dropdown list of flow input and output type.
Example: struct:$MQTTMessage

To Reproduce
Steps to reproduce the behavior:
Using 0.23.1:
Create a widget.
Start defining flow input or output

Expected behavior
(system) structs should be listed. User defined structs are listed.

Screenshots

Desktop (please complete the following information):

• OS: macOS 15.4

** LVGL version (if used)**
Both 8.x and 9.x

Additional context
Discussed on Discord