chore: ntx builder actor deactivation

[SantiagoPittella]

Closes the third task of #1694

• Add actor sterility timeout: default 5 min
• Re-activate deactivated actors when new notes target them via send_targeted, which now returns inactive target account IDs so the builder can re-spawn actors.
• Add coordinator unit tests for shutdown approval/rejection and targeted notification routing.
• Improves a bit the tests helpers org.

[Mirko-von-Leipzig]

I understand that this is following the established pattern, but I think we're needlessly complicating matters.

If the actor wants to exit, just exit the task. We don't actually need a shutdown handle or token either.

ActorShutdownReason can be replaced with a conventional -> Result<(), Error>. This may complicate this PR, so I'm okay with simply returning, and cleaning that up separately.

Suggested change

	// Sterility timeout: actor has been idle too long, request shutdown.
	_ = sterility_sleep => {
	match self.initiate_shutdown(account_id).await {
	Ok(()) => return ActorShutdownReason::Sterile(account_id),
	Err(()) => self.mode = ActorMode::NotesAvailable,
	}
	}
	// Sterility timeout: actor has been idle too long, deactivate account
	_ = sterility_sleep => return ActorShutdownReason::Sterile,

[Mirko-von-Leipzig]

I would avoid this entirely. Let the actor deactivate itself.

In the select! arm that reaps actor tasks, we can then check if the deactivated actor should be restarted by inspecting that actor's notify channel (which we have a copy of of here). If there is a pending notification, then we know the actor didn't handle it.

This should be rare because it implies the actor idle timed out just as we notified it. And therefore, even though this code is technically more optimal because the actor doesn't have to respawn, I wouldn't be concerned about the performance angle.

[SantiagoPittella]

Ok, sounds good. I will address this with the previous comment too