CVE-2026-106492: Backstage: Improper preservation of access restrictions during service credential delegation

### Impact An external service credential configured with access restrictions (e.g., read-only) could bypass those restrictions by routing requests through plugin delegation paths. This could allow a restricted service to perform operations beyond its intended scope, including write operations on plugins it was restricted to read-only access for. ### Patches Patched in `@backstage/backend-defaults` version `0.17.8` ### Workarounds If you're unable to upgrade immediately: - If practical, replace restricted credentials with separate, purpose-specific unrestricted credentials scoped to trusted consumers. - Restrict network-level access to Backstage backend API endpoints to trusted callers only.