Set up ad account billing (monthly invoicing)

Bills a Google Ads account to a monthly invoicing payments account of a manager above it, and proposes its account budget, so the account can serve without anyone opening Google Ads.

Monthly invoicing only. Google's API cannot add a card or other payment method, so paymentsAccountId must be one of the manager's monthly invoicing (consolidated billing) payments accounts listed by GET /v1/ads/accounts/billing. A manager without monthly invoicing has none, and billing has to be added in Google Ads instead.

Creates a billing setup (Google's BillingSetupService) that starts now on that payments account, then an account budget proposal that starts now and never ends: unlimited by default, or capped at spendingLimit in the account currency. Google reviews the budget offline, usually within an hour; until then the setup reads APPROVED_HELD and ads do not serve. A customer whose current billing setup already has a budget is refused with 409, and so is one billed to another payments account: this call never replaces existing billing. A current setup on the same payments account without a budget (for example after a budget rejection) is reused, so resending the same request finishes it.

Not idempotent and never retried: after a 502 with details.creationStatus=unknown, read GET /v1/ads/accounts/billing before retrying. Google has no dry run for billing setups, so validateOnly: true checks the payments account and the existing billing without creating anything (and asks Google to validate the budget when a setup would be reused).