UpdateCallbacks not accesible in testsuite

I’m trying to test my implementation of an UpdateHandler but I cannot find where UpdateCallbacks interface is defined (It won’t allow me to use the “internal” package in my test class).
I searched through SDK docs and the forum but I couldn’t find info about it.
I’m using go sdk version 1.23.1

In case someone encounters the same problem, here is an example of an UpdateHandler test.