fix: align HostPaymentTopUpError SCALE indices with triangle-js-sdks (#223)

PartialPayment is index 2 and Unknown is index 3, matching the
PaymentTopUpErr variant order in triangle-js-sdks.

Author: filvecchiato

rust/crates/truapi/src/v01/payment.rs

@@ -128,13 +128,13 @@ pub enum HostPaymentTopUpError {
     InsufficientFunds,
     /// The source account was not found or is invalid.
     InvalidSource,
-    /// Catch-all.
-    Unknown { reason: String },
     /// Some coins were claimed but the total fell short of the requested amount.
     PartialPayment {
         /// Amount that was successfully credited.
         credited: Balance,
     },
+    /// Catch-all.
+    Unknown { reason: String },
 }
 
 /// Error from [`crate::api::Payment::request`].