List read-only invoice send gates

Returns external invoice-send gates for all platform launch verticals when the authenticated automation client has platform.billing_owner_visibility.read. External URL: /v1/providers/current/invoice-send-gates. Current Next.js implementation path: /api/platform/v1/providers/current/invoice-send-gates. This read-only route does not require Idempotency-Key; it exposes finance-review prerequisites, dry-run invoice readiness, blocked delivery/payment/quota/credential side effects, and next operator actions only. It never reads usage event tables or partner-facing billing tables, returns raw usage payloads or raw payment payloads, sends invoices, collects payments, enforces quota, suspends credentials, or calls external finance providers.