Single source of truth for the compile-time fault-injection switch, shared by the producer (outgoing) and worker (incoming) to simulate a network partition.

In production builds ok?/0 is a hardcoded true that no runtime setting can flip, so injected faults can never activate. The switch is enabled under :test, or when a consuming app opts in for a demo:

# config/config.exs
config :echo_pubsub, :enable_fault_injection, true

Once enabled, ok?/0 honors the runtime :fault_injection flag, toggled with Application.put_env(:echo_pubsub, :fault_injection, :error | :ok).