Rename a subscription and/or change its billing country

Edits the subscription's display name (takes effect immediately; the id is frozen) and/or its billingCountry. A billing-country change takes effect from the NEXT billing period (first day of next month, UTC): every period already started keeps its country, closed invoices keep their frozen subsidiary snapshot, and a second change before then replaces the pending one. The response carries the country in force now and the scheduled pendingBillingCountry. Authorization: the subscription must be reached by the caller inside tenantId (else 404) and the caller must hold a SUBSCRIPTION-level owner grant on it -- a subscription grant or a tenant grant (else 403; owning a project beneath it is not enough). An API token is refused 403 TOKEN_SCOPE_MISSING (iam:write is never grantable). A tenant on non-payment hold is refused 403 TENANT_SUSPENDED. Validation: at least one of name / billingCountry (an empty body is 422); an unknown field is 422 (never silently ignored). name follows the create rule (ASCII, a letter or digit, 422 field=name) and must be unique (case-insensitive) among the tenant's subscriptions (409 CONFLICT, field=name). billingCountry must match ^[A-Z]{2}$ and be an allowed country (422 field=billingCountry, details.allowedBillingCountries). Every refusal happens before any write. Async & state: Synchronous; no Operation. Errors: 401; 403; 404; 409 name clash; 422 validation; 500. Idempotency/Concurrency: Naturally idempotent -- setting the current value again is a no-op success; no Idempotency-Key is read.