fix: add ShouldFail to Square MockClient for testing payment error paths

backend/internal/square/square_dev_test.go

@@ -361,6 +361,49 @@ func TestDevClient_CreateCardOnFileRaw_UnknownBrand(t *testing.T) {
 	assert.True(t, card.IsDefault)
 }
 
+func TestCreatePayment_ShouldFail(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	client.ShouldFail = true
+
+	ctx := context.Background()
+	req := CreatePaymentReq{
+		Amount:         5000,
+		Currency:       "GBP",
+		SourceID:       "cnon:test-card",
+		IdempotencyKey: "test-key-fail",
+		ReferenceID:    "booking-fail",
+	}
+
+	result, err := client.CreatePayment(ctx, req)
+	if err == nil {
+		t.Fatal("expected error when ShouldFail is true, got nil")
+	}
+	if result != nil {
+		t.Errorf("expected nil result, got %+v", result)
+	}
+}
+
+func TestRefundPayment_ShouldFail(t *testing.T) {
+	client := NewDevClient().(*MockClient)
+	client.ShouldFail = true
+
+	ctx := context.Background()
+	req := RefundPaymentReq{
+		PaymentID:      "pay_mock_fail",
+		Amount:         5000,
+		IdempotencyKey: "refund-key-fail",
+		Reason:         "simulated failure",
+	}
+
+	result, err := client.RefundPayment(ctx, req)
+	if err == nil {
+		t.Fatal("expected error when ShouldFail is true, got nil")
+	}
+	if result != nil {
+		t.Errorf("expected nil result, got %+v", result)
+	}
+}
+
 func TestDevClient_ConcurrentPayments(t *testing.T) {
 	client := NewDevClient().(*MockClient)