MC-19194: UpdateCartItems mutation does not update cart item quantity

- Reverted back commit c7d9130 and modified functional test

Author: Prabhu Ram

app/code/Magento/QuoteGraphQl/Model/Cart/UpdateCartItem.php

@@ -117,6 +117,7 @@ private function updateItemQuantity(int $itemId, Quote $cart, float $quantity)
         }
         $cartItem->setQty($quantity);
         $this->validateCartItem($cartItem);
+        $this->cartItemRepository->save($cartItem);
     }
 
     /**

dev/tests/api-functional/testsuite/Magento/GraphQl/Quote/Guest/UpdateCartItemsTest.php

@@ -70,6 +70,16 @@ public function testUpdateCartItemQuantity()
 
         $this->assertEquals($itemId, $item['id']);
         $this->assertEquals($quantity, $item['quantity']);
+
+        $cartQuery = $this->getCartQuery($maskedQuoteId);
+        $response = $this->graphQlQuery($cartQuery);
+
+        $this->assertArrayHasKey('cart', $response);
+
+        $responseCart = $response['cart'];
+        $item = current($responseCart['items']);
+
+        $this->assertEquals($quantity, $item['quantity']);
     }
 
     /**
@@ -268,6 +278,26 @@ private function getQuery(string $maskedQuoteId, int $itemId, float $quantity):
     }
   }
 }
+QUERY;
+    }
+
+    /**
+     * @param string $maskedQuoteId
+     * @return string
+     */
+    private function getCartQuery(string $maskedQuoteId)
+    {
+        return <<<QUERY
+query {
+  cart(cart_id: "{$maskedQuoteId}"){
+    items{
+      product{
+        name
+      }
+      quantity
+    }
+  }
+}
 QUERY;
     }
 }