Review fake_exponential function bounds

[fedacking]

The fake_exponential function needs to be reviews, as there are currently 3 different implementations (fake_exponential in block.rs, fake_exponential_checked in block.rs and fake_exponential in levms utils.rs) and they can all overflow based on different possible inputs. We should introduce an explicit bound based on the usage of the function (calculating blob fee) and add a test for that scenario.