Refactor `handlers` to be sync-agnostic

[GitGab19]

The current handlers trait is heavily tied to Mutex-based synchronization.

Since this trait is part of roles_logic_sv2, and it's intended for building custom roles, it shouldn't enforce any specific sync strategy.

We’ve recently improved the channel modules to be agnostic in this regard (see #1630) , and handlers should follow the same principle imo.

The trait should define behavior, not implementation constraints like synchronization.