feat(cart): implement optimistic quantity updates with rollback on error

src/app/shared/components/cart/cart.component.spec.ts

@@ -6,7 +6,7 @@ import {
   BrowserTestingModule,
   platformBrowserTesting
 } from '@angular/platform-browser/testing';
-import { of } from 'rxjs';
+import { of, throwError } from 'rxjs';
 import { afterEach, beforeAll, describe, expect, it, vi } from 'vitest';
 
 import { CartService } from '../../../core/services/cart/cart.service';
@@ -141,4 +141,136 @@ describe('CartComponent', () => {
     expect(openConfirmDelete).toHaveBeenCalled();
     expect(removeItem).not.toHaveBeenCalled();
   });
+
+  it('optimistically updates quantity and rolls back on error', async () => {
+    vi.useFakeTimers();
+    const updateItemQuantity = vi.fn().mockReturnValue(throwError(() => new Error('Error')));
+    const danger = vi.fn();
+
+    await TestBed.configureTestingModule({
+      imports: [CartComponent],
+      providers: [
+        {
+          provide: CartService,
+          useValue: {
+            cart: signal(null).asReadonly(),
+            updateItemQuantity,
+            removeItem: vi.fn()
+          }
+        },
+        {
+          provide: ModalService,
+          useValue: {}
+        },
+        {
+          provide: ToastService,
+          useValue: {
+            success: vi.fn(),
+            info: vi.fn(),
+            danger
+          }
+        }
+      ]
+    }).compileComponents();
+
+    const fixture = TestBed.createComponent(CartComponent);
+    const component = fixture.componentInstance;
+
+    const item = {
+      productVariantId: 10,
+      imageUrl: null,
+      product: 'Producto de prueba',
+      originalPrice: null,
+      discountedPrice: 1000,
+      discountPercentage: null,
+      attributes: [],
+      quantity: 1
+    };
+
+    fixture.componentRef.setInput('items', [item]);
+    fixture.detectChanges();
+
+    // Trigger quantity change to 3
+    fixture.debugElement.query(By.css('app-cart-item')).triggerEventHandler('quantityChange', 3);
+    fixture.detectChanges();
+
+    // Optimistic update should be active immediately in local getter
+    expect((component as any).getItemQuantity(item)).toBe(3);
+
+    // Wait for the debounce time (1000ms)
+    vi.advanceTimersByTime(1000);
+    fixture.detectChanges();
+
+    // After failure, it should roll back to original quantity (1)
+    expect((component as any).getItemQuantity(item)).toBe(1);
+    expect(danger).toHaveBeenCalled();
+
+    vi.useRealTimers();
+  });
+
+  it('optimistically updates quantity and clears override on success', async () => {
+    vi.useFakeTimers();
+    const updateItemQuantity = vi.fn().mockReturnValue(of({ message: 'Success', data: {} }));
+    const success = vi.fn();
+
+    await TestBed.configureTestingModule({
+      imports: [CartComponent],
+      providers: [
+        {
+          provide: CartService,
+          useValue: {
+            cart: signal(null).asReadonly(),
+            updateItemQuantity,
+            removeItem: vi.fn()
+          }
+        },
+        {
+          provide: ModalService,
+          useValue: {}
+        },
+        {
+          provide: ToastService,
+          useValue: {
+            success,
+            info: vi.fn(),
+            danger: vi.fn()
+          }
+        }
+      ]
+    }).compileComponents();
+
+    const fixture = TestBed.createComponent(CartComponent);
+    const component = fixture.componentInstance;
+
+    const item = {
+      productVariantId: 10,
+      imageUrl: null,
+      product: 'Producto de prueba',
+      originalPrice: null,
+      discountedPrice: 1000,
+      discountPercentage: null,
+      attributes: [],
+      quantity: 1
+    };
+
+    fixture.componentRef.setInput('items', [item]);
+    fixture.detectChanges();
+
+    // Trigger quantity change to 3
+    fixture.debugElement.query(By.css('app-cart-item')).triggerEventHandler('quantityChange', 3);
+    fixture.detectChanges();
+
+    // Optimistic update should be active immediately in local getter
+    expect((component as any).getItemQuantity(item)).toBe(3);
+
+    // Wait for the debounce time (1000ms)
+    vi.advanceTimersByTime(1000);
+    fixture.detectChanges();
+
+    // After success, it should clear override and use input quantity (which is 1 since we didn't update items input here)
+    expect((component as any).getItemQuantity(item)).toBe(1);
+    expect(success).toHaveBeenCalled();
+
+    vi.useRealTimers();
+  });
 });