Update sandbox bank application status dry-run

Returns deterministic sandbox status-transition dry-run metadata for a bank application when the authenticated automation client has banks.applications.status.write. External URL: /v1/banks/current/applications/{id}/status. Current Next.js implementation path: /api/platform/v1/banks/current/applications/[id]/status. This sandbox-only route requires Idempotency-Key, enable_platform_bank_frontends, and enable_platform_bank_workflow_write; it records replay-safe transition metadata only and never persists application status, writes a bank system, calls providers, exposes bank secrets, exposes raw MyInfo, exposes credit bureau data, exposes bank statements, exposes raw document contents, exposes NRIC/FIN, exposes income-proof contents, sends callbacks, sends borrower notifications, returns signed storage URLs, or claims API Gateway/enterprise ingress readiness. Live bank status writes remain unavailable until promotion gates have REST, consent, audit, sandbox, callback, operations, and enterprise-ingress evidence. Bank pilot posture remains information/referral and pre-approval only, recommendation and live bank-decision claims remain deferred, and partner-status publication remains private-first.