feat(abstract-utxo): improve parseTransaction expected outputs logic

Move transaction explanation logic after expected outputs calculation to
improve logical flow. Change how external change addresses are handled in
expectedOutputs to use 'max' amount instead of adding all outputs.

Issue: BTC-2732

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

modules/abstract-utxo/src/transaction/fixedScript/parseTransaction.ts

@@ -54,8 +54,7 @@ function toExpectedOutputs(
     recipients?: ITransactionRecipient[];
     allowExternalChangeAddress?: boolean;
     changeAddress?: string;
-  },
-  allOutputs: Output[]
+  }
 ): Output[] {
   // verify that each recipient from txParams has their own output
   const expectedOutputs = (txParams.recipients ?? []).flatMap((output) => {
@@ -70,17 +69,7 @@ function toExpectedOutputs(
   if (txParams.allowExternalChangeAddress && txParams.changeAddress) {
     // when an external change address is explicitly specified, count all outputs going towards that
     // address in the expected outputs (regardless of the output amount)
-    expectedOutputs.push(
-      ...allOutputs.flatMap((output) => {
-        if (
-          output.address === undefined ||
-          output.address !== coin.canonicalAddress(txParams.changeAddress as string)
-        ) {
-          return [];
-        }
-        return [{ ...output, address: coin.canonicalAddress(output.address) }];
-      })
-    );
+    expectedOutputs.push({ address: coin.canonicalAddress(txParams.changeAddress), amount: 'max' });
   }
   return expectedOutputs;
 }
@@ -120,16 +109,7 @@ export async function parseTransaction<TNumber extends bigint | number>(
     throw new Error('missing required txPrebuild property txHex');
   }
 
-  // obtain all outputs
-  const explanation: TransactionExplanation = await coin.explainTransaction<TNumber>({
-    txHex: txPrebuild.txHex,
-    txInfo: txPrebuild.txInfo,
-    pubs: keychainArray.map((k) => k.pub) as Triple<string>,
-  });
-
-  const allOutputs = [...explanation.outputs, ...explanation.changeOutputs];
-
-  const expectedOutputs = toExpectedOutputs(coin, txParams, allOutputs);
+  const expectedOutputs = toExpectedOutputs(coin, txParams);
 
   // get the keychains from the custom change wallet if needed
   let customChange: CustomChangeOptions | undefined;
@@ -159,6 +139,15 @@ export async function parseTransaction<TNumber extends bigint | number>(
     }
   }
 
+  // obtain all outputs
+  const explanation: TransactionExplanation = await coin.explainTransaction<TNumber>({
+    txHex: txPrebuild.txHex,
+    txInfo: txPrebuild.txInfo,
+    pubs: keychainArray.map((k) => k.pub) as Triple<string>,
+  });
+
+  const allOutputs = [...explanation.outputs, ...explanation.changeOutputs];
+
   /**
    * Loop through all the outputs and classify each of them as either internal spends
    * or external spends by setting the "external" property to true or false on the output object.