List the subscription's billing contracts (read-only history)

Returns the customer contracts of the active scope's subscription, newest-first, each a BillingContract with its per-SKU override lines nested, in the standard { data, meta } envelope. Keyed on the billing grain (scope_id == subscription_id). READ-ONLY: contracts are operator-managed (removed the tenant create / add-line / activate writes); historical tenant-written contracts stay listable here (CANCELLED) and are never priced. Validation: scopeId must resolve to a scope the caller can reach (unknown → 404 NOT_FOUND); page ≥ 1 and pageSize within 1..200 → else 422 VALIDATION_FAILED. RBAC: READ on billing plus the billing:read token scope, held at SUBSCRIPTION level — a grant on the path project alone is refused 403 (details.reason SUBSCRIPTION_ROLE_REQUIRED). Async & state: Synchronous read; no state change. Errors: 401 unauthenticated; 403 caller lacks subscription-level billing READ or the token scope; 404 unknown scopeId; 422 malformed pagination; 500 internal failure. Idempotency/Concurrency: Safe, cacheable; no idempotency key required.