Add test for cart deactivation on crypto payment confirmation

Added test_finalize_invoice_deactivates_cart to verify that when a
cryptocurrency payment is confirmed, the user's active cart is replaced
with a new empty cart.

The test verifies:
- User starts with a non-empty cart
- After finalize_invoice(), user gets a new empty cart
- New cart has different ID than the old cart

This ensures the UX improvement works correctly - users see an empty
cart after their crypto payment is confirmed.

make_post_sell/tests/test_crypto_watcher.py

@@ -310,6 +310,62 @@ class CryptoWatcherIntegrationTests(unittest.TestCase):
             # Check inventory was deducted
             self.assertEqual(inventory.quantity, 7)  # 10 - 3
 
+    @patch("make_post_sell.lib.crypto_watcher.send_purchase_email")
+    @patch("make_post_sell.lib.crypto_watcher.send_sale_email")
+    def test_finalize_invoice_deactivates_cart(self, mock_sale_email, mock_purchase_email):
+        """Test that finalizing invoice creates new empty cart for user."""
+        with transaction.manager:
+            dbsession = get_tm_session(self.session_factory, transaction.manager)
+            
+            # Get objects from database
+            user = dbsession.query(User).filter_by(id=self.user_id).first()
+            shop = dbsession.query(Shop).filter_by(id=self.shop_id).first()
+            product = dbsession.query(Product).filter_by(id=self.product_id).first()
+            
+            # Create and set up an active cart with products
+            old_cart = shop.create_new_cart_for_user(user)
+            old_cart.add_product(product)
+            dbsession.add(old_cart)
+            dbsession.flush()
+            
+            # Verify cart has products
+            self.assertFalse(old_cart.is_empty)
+            old_cart_id = old_cart.id
+            
+            # Create invoice and payment
+            invoice = Invoice(user)
+            invoice.shop = shop
+            invoice.new_line_item(product=product, quantity=1)
+            dbsession.add(invoice)
+            
+            payment = CryptoPayment(
+                invoice=invoice,
+                address="test_cart_address",
+                account_index=0,
+                subaddress_index=1,
+                coin_type="XMR",
+                expected_amount=1000000000000,  # 1 XMR in atomic units
+                rate_locked_usd_per_coin=Decimal("150.00"),
+                quote_expires_at_ms=int(time.time() * 1000) + 900000,
+            )
+            dbsession.add(payment)
+            dbsession.flush()
+            
+            # Create mock request
+            mock_request = MagicMock()
+            mock_request.dbsession = dbsession
+            mock_request.registry.settings = {"app.email.enabled": "true"}
+            
+            # Finalize the invoice
+            finalize_invoice(mock_request, payment)
+            dbsession.flush()
+            
+            # Check that a new cart was created
+            new_cart = shop.get_active_cart_for_user(user)
+            self.assertIsNotNone(new_cart)
+            self.assertNotEqual(new_cart.id, old_cart_id)
+            self.assertTrue(new_cart.is_empty)
+
     def test_process_payment_no_transfers(self):
         """Test processing payment with no incoming transfers."""
         with transaction.manager: