Automation-controller: automation-controller: notification.recipients/subject/error lack prevent_search, allowing zero-privilege cross-tenant recovery of notification recipient secrets via filter oracle

A flaw was found in the automation-controller notification subsystem. Although NotificationTemplate.notification_ configuration is protected from API filtering, its recipient value is copied in clear text into the unprotected Notification.recipients field on every send. Because the credential-types endpoint is listable by any authenticated user and the API filter backend traverses object relations without per-hop authorization, a user with no privileges can use a relational filter as a boolean count-oracle to recover, character by character and across organizations, the secret recipient values of other tenants' notifications — including PagerDuty service keys and Slack/Mattermost/RocketChat/Webhook bearer-token URLs. This flaw affects confidentiality.