feat(abstract-utxo): add test for explainPsbt utility

Add unit test for the explainPsbt utility function using AcidTest framework
from utxo-lib. Tests ensure that transaction explanations correctly include
output amounts, change outputs, and signature counts across different signing
stages.

Issue: BTC-2732

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

Author: OttoAllmendinger

modules/abstract-utxo/test/unit/transaction/fixedScript/explainPsbt.ts

@@ -0,0 +1,41 @@
+import assert from 'node:assert/strict';
+
+import { testutil } from '@bitgo/utxo-lib';
+
+import { explainPsbt } from '../../../../src/transaction/fixedScript';
+
+function describeTransactionWith(acidTest: testutil.AcidTest) {
+  describe(`explainPsbt ${acidTest.name}`, function () {
+    it('should explain the transaction', function () {
+      const psbt = acidTest.createPsbt();
+      const explanation = explainPsbt(
+        psbt,
+        { pubs: acidTest.rootWalletKeys.triple.map((k) => k.toBase58()) },
+        acidTest.network,
+        { strict: true }
+      );
+      assert.strictEqual(explanation.outputs.length, 3);
+      assert.strictEqual(explanation.outputAmount, '2700');
+      assert.strictEqual(explanation.changeOutputs.length, acidTest.outputs.length - 3);
+      explanation.changeOutputs.forEach((change) => {
+        assert.strictEqual(change.amount, '900');
+        assert.strictEqual(typeof change.address, 'string');
+      });
+      assert.strictEqual(explanation.inputSignatures.length, acidTest.inputs.length);
+      explanation.inputSignatures.forEach((signature, i) => {
+        if (acidTest.inputs[i].scriptType === 'p2shP2pk') {
+          return;
+        }
+        if (acidTest.signStage === 'unsigned') {
+          assert.strictEqual(signature, 0);
+        } else if (acidTest.signStage === 'halfsigned') {
+          assert.strictEqual(signature, 1);
+        } else if (acidTest.signStage === 'fullsigned') {
+          assert.strictEqual(signature, 2);
+        }
+      });
+    });
+  });
+}
+
+testutil.AcidTest.suite().forEach((test) => describeTransactionWith(test));