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.