Function throttler_resume 

pub fn throttler_resume<T, F>(actor_id: Ustr) -> TimeEventCallback
where
    T: 'static + Debug,
    F: Fn(T) + 'static,

Sets throttler to resume sending messages.

Uses try_get_actor_unchecked so that embedded throttlers (not registered in the actor registry) are handled gracefully. The send() auto-reset ensures such throttlers recover once the rate window passes.

When messages are buffered (possible after a rejected try_reserve), the throttler stays limiting and arms its timer with the drain callback instead, so buffered messages drain in FIFO order ahead of later sends.