[lldb][api-test] Add API test for SBCommandInterpreter::CommandOverrideCallback (#94518)

`SBCommandInterpreter::CommandOverrideCallback` was not being exposed to
the Python API and has no coverage in the
API test suite, so this commits exposes and adds a test for it. Doing
this involves also adding a typemap for the callback used for this
function so that it matches the functionality of other callback
functions that are exposed to Python.