feat(abstract-utxo): refactor PSBT output handling

Improve type safety for PSBT outputs by creating dedicated type guards and
helper functions for wallet and external outputs. This makes the code more
maintainable and adds explicit error handling for invalid output types.

Issue: BG-62732

Co-authored-by: llm-git <llm-git@ttll.de>

Author: OttoAllmendinger

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

@@ -9,6 +9,35 @@ function scriptToAddress(script: Uint8Array): string {
   return `scriptPubKey:${Buffer.from(script).toString('hex')}`;
 }
 
+type ParsedWalletOutput = fixedScriptWallet.ParsedOutput & { scriptId: fixedScriptWallet.ScriptId };
+type ParsedExternalOutput = fixedScriptWallet.ParsedOutput & { scriptId: null };
+
+function isParsedWalletOutput(output: ParsedWalletOutput | ParsedExternalOutput): output is ParsedWalletOutput {
+  return output.scriptId !== null;
+}
+
+function isParsedExternalOutput(output: ParsedWalletOutput | ParsedExternalOutput): output is ParsedExternalOutput {
+  return output.scriptId === null;
+}
+
+function toChangeOutput(output: ParsedWalletOutput): FixedScriptWalletOutput {
+  return {
+    address: output.address ?? scriptToAddress(output.script),
+    amount: output.value.toString(),
+    chain: output.scriptId.chain,
+    index: output.scriptId.index,
+    external: false,
+  };
+}
+
+function toExternalOutput(output: ParsedExternalOutput): Output {
+  return {
+    address: output.address ?? scriptToAddress(output.script),
+    amount: output.value.toString(),
+    external: true,
+  };
+}
+
 export function explainPsbtWasm(
   psbt: fixedScriptWallet.BitGoPsbt,
   walletXpubs: Triple<string>,
@@ -25,24 +54,14 @@ export function explainPsbtWasm(
   const outputs: Output[] = [];
 
   parsed.outputs.forEach((output) => {
-    const address = output.address ?? scriptToAddress(output.script);
-
-    if (output.scriptId) {
+    if (isParsedWalletOutput(output)) {
       // This is a change output
-      changeOutputs.push({
-        address,
-        amount: output.value.toString(),
-        chain: output.scriptId.chain,
-        index: output.scriptId.index,
-        external: false,
-      });
-    } else {
+      changeOutputs.push(toChangeOutput(output));
+    } else if (isParsedExternalOutput(output)) {
       // This is an external output
-      outputs.push({
-        address,
-        amount: output.value.toString(),
-        external: true,
-      });
+      outputs.push(toExternalOutput(output));
+    } else {
+      throw new Error('Invalid output');
     }
   });