Get a Stripe-hosted billing page where the card on file is updated.

HTTP-only (not an MCP tool). Send an empty body: it is never read, and the Stripe customer is always the caller's own, so no body field can name another account. Returns the URL of a one-time Stripe-hosted page; the page's return address is the dashboard Billing page. 409 no_stripe_customer means the account has no Stripe billing record yet (go live first). 502 billing_portal_unavailable means Stripe could not open the page right now; the Stripe text is not passed through.