fix: dev mock parity — GetCheckout/GetPayment return structured NOT_FOUND errors matching prod

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/internal/square/square_dev.go

@@ -1165,7 +1165,13 @@ func (m *MockClient) GetCheckout(ctx context.Context, checkoutID string) (*Payme
 
 	checkout, ok := m.checkouts[checkoutID]
 	if !ok {
-		return nil, fmt.Errorf("checkout not found: %s", checkoutID)
+		return nil, &squareAPIError{
+			Code:       "NOT_FOUND",
+			Category:   "INVALID_REQUEST_ERROR",
+			Detail:     "checkout not found",
+			StatusCode: http.StatusNotFound,
+			err:        fmt.Errorf("square: checkout not found: %s", checkoutID),
+		}
 	}
 
 	// Mirror the real client's GetCheckout state machine
@@ -1181,7 +1187,13 @@ func (m *MockClient) GetCheckout(ctx context.Context, checkoutID string) (*Payme
 	case "COMPLETED":
 		result, ok := m.completed[checkoutID]
 		if !ok {
-			return nil, fmt.Errorf("checkout result not found: %s", checkoutID)
+			return nil, &squareAPIError{
+				Code:       "NOT_FOUND",
+				Category:   "INVALID_REQUEST_ERROR",
+				Detail:     "checkout result not found",
+				StatusCode: http.StatusNotFound,
+				err:        fmt.Errorf("square: checkout result not found: %s", checkoutID),
+			}
 		}
 		return result, nil
 	default:
@@ -1197,7 +1209,13 @@ func (m *MockClient) GetPayment(ctx context.Context, paymentID string) (*Payment
 
 	payment, ok := m.payments[paymentID]
 	if !ok {
-		return nil, fmt.Errorf("payment not found: %s", paymentID)
+		return nil, &squareAPIError{
+			Code:       "NOT_FOUND",
+			Category:   "INVALID_REQUEST_ERROR",
+			Detail:     "payment not found",
+			StatusCode: http.StatusNotFound,
+			err:        fmt.Errorf("square: payment not found: %s", paymentID),
+		}
 	}
 	return payment, nil
 }