SMS transport — delegates to a consumer-configured backend.

Configure a backend module in your app config and ash_dispatch will call its deliver/4 callback every time an event with an :sms channel fires:

config :ash_dispatch, :sms_backend, MyApp.SMS

The backend module must implement AshDispatch.SMSBackend — essentially one function:

defmodule MyApp.SMS do
  @behaviour AshDispatch.SMSBackend

  @impl true
  def deliver(receipt, context, channel, event_config) do
    # send via Twilio/Telavox/etc.
    # then update the receipt: status :sent (success) or :failed.
    # return `{:ok, updated_receipt}` or `{:error, reason}`.
  end
end

When no backend is configured the receipt is marked :skipped with error_message: "transport_not_implemented" — same as the original stub. This preserves backwards-compat for consumers that haven't wired SMS yet.