feat: remove usage of 'invoice' in amount field

modules/sdk-core/src/coins/ofcToken.ts

@@ -10,7 +10,7 @@ import {
   SignedTransaction,
   ITransactionRecipient,
 } from '../';
-import { isBolt11Invoice, LIGHTNING_INVOICE } from '../lightning';
+import { isBolt11Invoice } from '../lightning';
 
 import { Ofc } from './ofc';
 
@@ -76,25 +76,19 @@ export class OfcToken extends Ofc {
         throw new Error(`invalid argument - lightning invoice is only supported for bitcoin`);
       }
 
-      // amount for bolt11 invoices is either 'invoice' or a non-zero bigint
-      if (recipient.amount === LIGHTNING_INVOICE) {
-        return;
-      }
-      // try to parse the amount as a bigint
+      // amount for bolt11 invoices must be a non-zero bigint
       let amount: bigint;
       try {
         amount = BigInt(recipient.amount);
       } catch (e) {
         throw new Error(
-          `invalid argument ${recipient.amount} for amount - lightning invoice amount must be >= 0 or ${LIGHTNING_INVOICE}`
+          `invalid argument ${recipient.amount} for amount - lightning invoice amount must be a non-zero bigint`
         );
       }
       if (amount > 0n) {
         return;
       }
-      throw new Error(
-        `invalid argument for amount - lightning invoice amount must be a non-zero bigint or ${LIGHTNING_INVOICE}`
-      );
+      throw new Error(`invalid argument for amount - lightning invoice amount must be a non-zero bigint`);
     }
 
     super.checkRecipient(recipient);

modules/sdk-core/src/lightning.ts

@@ -1,5 +1,3 @@
-export const LIGHTNING_INVOICE = 'invoice';
-
 export function isBolt11Invoice(value: unknown): value is string {
   if (typeof value !== 'string') {
     return false;