fix: replace 3.5s sleep with poll loop in till checkout test

backend/handlers/payments/till_test.go

@@ -540,10 +540,18 @@ func TestGetTillCheckoutStatus_Completed(t *testing.T) {
 		t.Fatal("expected checkout_id to be set for card_machine payment")
 	}
 
-	// Wait for the mock goroutine to complete. The mock's goroutine sleeps 3s
-	// by default (mockSleep is only skipped when isTesting is set before the
-	// square package initializes, which depends on init ordering with db).
-	time.Sleep(3500 * time.Millisecond)
+	// Wait for the mock goroutine to complete the checkout with polling.
+	deadline := time.Now().Add(5 * time.Second)
+	for {
+		_, err := SquareClient.GetCheckout(context.Background(), *createResp.CheckoutID)
+		if err == nil {
+			break
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("timed out waiting for checkout to complete: %v", err)
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
 
 	// Now call GetTillCheckoutStatus — should return COMPLETED.
 	statusReq := httptest.NewRequest("GET", "/api/admin/till/checkout/"+*createResp.CheckoutID+"/status", nil)