Rejects a booked trade the way a broker fill rejection would, driving the same domain command the real broker integration sends, so client integrations can exercise the failed-trade flow without a live counterparty. The trade ends Failed with the broker-rejection reason, observable through the standard trade reads. Sandbox-only: the scenario is registered exclusively when the platform runs in Simulated integrations mode, so in Real or Mocked mode this endpoint returns 404 for every request. Only a booked trade can be rejected, and the sandbox broker fills every trade on initiation unless its internalReference starts with TRADE-pending, so place the trade with that trigger to keep it rejectable.