Retrieve the account-level billing subscription detail read model

Returns the account-level billing subscription read model that backs the subscription detail page (Overview / Resources / Cost breakdown / Invoices / Quotas / Activity tabs): the SubscriptionAccount record — GUID, lifecycle state, offer, spending mode, billing cycle day, payment-method label, finance owner, cost center, month-to-date / projected / last-month spend, forecast delta, credit balance, outstanding amount and optional prepaid spending limit (all money fields in ten-thousandths of the currency unit) — plus the subscription activity log entries shown on the Activity tab. The Resources, Cost, Invoices and Quotas panels are populated by their sibling endpoints (billing.billing_subscription.list, billing.cost_report/cost explorer reads, billing.invoice.list, billing.quota.list). Validation: billingSubscriptionId must identify a subscription visible to the caller within scopeId (else 404 NOT_FOUND — cross-tenant existence is never disclosed); the caller must hold at least the reader or billing role in the scope (else 403). Async & state: Synchronous read; no state change. No operation is created and no resource.updated SSE frames are emitted; state transitions caused by pause/resume commands arrive as kind 'billingSubscription' frames produced by those commands, not by this read. Errors: 401 missing/expired session; 403 role lacks billing read access in this scope; 404 unknown or invisible billingSubscriptionId; 500 internal failure assembling the read model. Idempotency/Concurrency: Safe, cacheable; no idempotency key required. The response carries no ETag; pause does not check If-Match.