fix: invalid unit comparison in lnurl tests

[claddyy]

boltz-rust/src/util/lnurl.rs

Line 139 in 348a881

Ok(invoice) => invoice.amount_milli_satoshis().unwrap() / 1000,

Here we were converting the amount from milli_satoshi to satoshi, but max_withdrawable returns us amount in satoshi. So it was fundamentally a wrong comparison.

boltz-rust/src/util/lnurl.rs

Line 147 in 348a881

invoice_amount <= withdraw_response.max_withdrawable,

nit: Also includes minor change in error message.