fix(core-flows): Avoid throwing if no prices found for variant when adding to cart custom price item (#14528)

* Avoid throwing prices not found error when line item is custom unit price

* Add changeset

* Avoid throwing upo variant price validation for custom priced item variants

integration-tests/modules/__tests__/cart/store/cart.workflows.spec.ts

@@ -2474,6 +2474,103 @@ medusaIntegrationTestRunner({
           ])
         })
 
+        it("should add item to cart with unit_price when variant has no price set", async () => {
+          const salesChannel = await scModuleService.createSalesChannels({
+            name: "Webshop",
+          })
+
+          const location = await stockLocationModule.createStockLocations({
+            name: "Warehouse",
+          })
+
+          let cart = await cartModuleService.createCarts({
+            currency_code: "usd",
+            sales_channel_id: salesChannel.id,
+          })
+
+          const [product] = await productModule.createProducts([
+            {
+              title: "Test product",
+              status: ProductStatus.PUBLISHED,
+              variants: [
+                {
+                  title: "Test variant",
+                },
+              ],
+            },
+          ])
+
+          const inventoryItem = await inventoryModule.createInventoryItems({
+            sku: "inv-1234",
+          })
+
+          await inventoryModule.createInventoryLevels([
+            {
+              inventory_item_id: inventoryItem.id,
+              location_id: location.id,
+              stocked_quantity: 2,
+              reserved_quantity: 0,
+            },
+          ])
+
+          await remoteLink.create([
+            {
+              [Modules.SALES_CHANNEL]: {
+                sales_channel_id: salesChannel.id,
+              },
+              [Modules.STOCK_LOCATION]: {
+                stock_location_id: location.id,
+              },
+            },
+            {
+              [Modules.PRODUCT]: {
+                variant_id: product.variants[0].id,
+              },
+              [Modules.INVENTORY]: {
+                inventory_item_id: inventoryItem.id,
+              },
+            },
+          ])
+
+          cart = await cartModuleService.retrieveCart(cart.id, {
+            select: ["id", "region_id", "currency_code", "sales_channel_id"],
+          })
+
+          // Add item with unit_price specified even though variant has no price set
+          await addToCartWorkflow(appContainer).run({
+            input: {
+              items: [
+                {
+                  variant_id: product.variants[0].id,
+                  quantity: 1,
+                  unit_price: 5000,
+                },
+              ],
+              cart_id: cart.id,
+            },
+          })
+
+          cart = await cartModuleService.retrieveCart(cart.id, {
+            relations: ["items"],
+          })
+
+          expect(cart).toEqual(
+            expect.objectContaining({
+              id: cart.id,
+              currency_code: "usd",
+              items: expect.arrayContaining([
+                expect.objectContaining({
+                  unit_price: 5000,
+                  is_custom_price: true,
+                  quantity: 1,
+                  title: "Test product",
+                  subtitle: "Test variant",
+                }),
+              ]),
+            })
+          )
+        })
+
         describe("setPricingContext hook", () => {
           it("should use context provided by the hook", async () => {
             const salesChannel = await scModuleService.createSalesChannels({