fix: address review feedback on InstrumentAmount migration

- Add round-trip assertions for PostingAmount fields in serialization test
- Deduplicate cents-to-InstrumentAmount conversion in grpc_lifecycle.go
  to use shared buildPostingAmount helper

Author: bjcoombs

services/payment-order/service/grpc_lifecycle.go

@@ -420,14 +420,7 @@ func (s *Service) reverseLedgerPosting(ctx context.Context, po *domain.PaymentOr
 
 	// Convert amount to InstrumentAmount format
 	amountCents := domain.ToMinorUnits(po.Amount)
-	majorUnits := amountCents / 100
-	minorUnits := amountCents % 100
-	amountStr := fmt.Sprintf("%d.%02d", majorUnits, minorUnits)
-	postingAmount := &quantityv1.InstrumentAmount{
-		Amount:         amountStr,
-		InstrumentCode: currencyCode,
-		Version:        1,
-	}
+	postingAmount := buildPostingAmount(currencyCode, amountCents)
 	valueDate := timestamppb.Now()
 
 	// Steps 2-3: Create CREDIT and DEBIT reversal postings

tests/proto/serialization_test.go

@@ -96,6 +96,18 @@ func TestProtoSerialization(t *testing.T) {
 		if decoded.PostingDirection != original.PostingDirection {
 			t.Errorf("posting_direction mismatch: got %v, want %v", decoded.PostingDirection, original.PostingDirection)
 		}
+		if decoded.PostingAmount == nil {
+			t.Fatal("posting_amount mismatch: got nil, want non-nil")
+		}
+		if decoded.PostingAmount.Amount != original.PostingAmount.Amount {
+			t.Errorf("posting_amount.amount mismatch: got %v, want %v", decoded.PostingAmount.Amount, original.PostingAmount.Amount)
+		}
+		if decoded.PostingAmount.InstrumentCode != original.PostingAmount.InstrumentCode {
+			t.Errorf("posting_amount.instrument_code mismatch: got %v, want %v", decoded.PostingAmount.InstrumentCode, original.PostingAmount.InstrumentCode)
+		}
+		if decoded.PostingAmount.Version != original.PostingAmount.Version {
+			t.Errorf("posting_amount.version mismatch: got %v, want %v", decoded.PostingAmount.Version, original.PostingAmount.Version)
+		}
 	})
 
 	t.Run("Event message serialization", func(t *testing.T) {