Read payee tax status

Privileged per-payee read returning completion state plus the flat withholding rate in integer basis points (stored 24% -> 2400). Requires a client secret carrying the payee_tax_detail scope. This is a server-to-server interface: no CORS headers are set, so it cannot be called from a browser. withholdingRateBps: null means no flat rate exists for the payee: either no completed submission, or a payment-dependent form family (8233) whose withholding cannot be expressed as a flat rate even after completion. The address 0x0000000000000000000000000000000000000001 is reserved for testing and always returns completed: true with a null rate.