PER_EXCHANGE rules: 400 if reNotifyMinutes != 0 or forDurationSeconds != 0. Any rule: 400 if webhooks + targets are both empty (never notifies anyone). Turns green: AlertRuleControllerIT#createPerExchangeRule_with*NonZero_returns400, AlertRuleControllerIT#createAnyRule_withEmptyWebhooksAndTargets_returns400.