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.