[frontend] `CleartextBackend` bug fix/cleanup

[AlexanderViand]

Looks like something broke in the recent frontend refactoring: TypeError: CleartextBackend.run_backend() missing 1 required positional argument: 'debug'

Also, there's a (commented out) example in frontend/example.py that passes backend=None to @compile(...) and says that'll use the CleartextBackend but this now actually dispatches to OpenFHEBackend, so needs to be updated.

Finally, it would probably be a good idea to (a) wrap each example in the example file in a func so that they can be un-commented at the same time, (b) add a test to ensure the examples don't randomly break again, (c) add specific CleartextBackend test