Migrate cursor rules (#12646)

Migrating rules to new format but they should be re-written entirely, I
don't think they help much and are not auto-included (except
architecture)

.cursor/rules/testing-guidelines.mdc

@@ -0,0 +1,89 @@
+---
+description: 
+globs: 
+alwaysApply: false
+---
+# Testing Guidelines
+
+## Test Structure (AAA Pattern)
+```typescript
+describe('UserService', () => {
+  describe('when getting user by ID', () => {
+    it('should return user data for valid ID', async () => {
+      // Arrange
+      const userId = '123';
+      const expectedUser = { id: '123', name: 'John' };
+      mockUserRepository.findById.mockResolvedValue(expectedUser);
+
+      // Act
+      const result = await userService.getUserById(userId);
+
+      // Assert
+      expect(result).toEqual(expectedUser);
+    });
+  });
+});
+```
+
+## React Component Testing
+```typescript
+// ✅ Test user behavior, not implementation
+describe('LoginForm', () => {
+  it('should display error message for invalid credentials', async () => {
+    const mockOnSubmit = jest.fn().mockRejectedValue(new Error('Invalid credentials'));
+    render(<LoginForm onSubmit={mockOnSubmit} />);
+
+    await user.type(screen.getByLabelText(/email/i), 'invalid@example.com');
+    await user.type(screen.getByLabelText(/password/i), 'wrongpassword');
+    await user.click(screen.getByRole('button', { name: /sign in/i }));
+
+    expect(await screen.findByText(/invalid credentials/i)).toBeInTheDocument();
+  });
+});
+```
+
+## Mocking Patterns
+```typescript
+// ✅ Service mocking
+const mockEmailService = {
+  sendEmail: jest.fn().mockResolvedValue({ success: true }),
+  validateEmail: jest.fn().mockReturnValue(true),
+};
+
+// ✅ Test data factories
+const createTestUser = (overrides = {}) => ({
+  id: uuid(),
+  email: 'test@example.com',
+  name: 'Test User',
+  ...overrides,
+});
+
+beforeEach(() => {
+  jest.clearAllMocks();
+});
+```
+
+## Testing Principles
+- **Test behavior, not implementation** - Focus on what users see/do
+- **Use descriptive test names** - "should [behavior] when [condition]"
+- **Query by user-visible elements** - text, roles, labels over test IDs
+- **Keep tests isolated** - Independent and repeatable
+- **70% unit, 20% integration, 10% E2E** - Test pyramid
+
+## Common Patterns
+```typescript
+// Async testing
+await waitFor(() => {
+  expect(screen.getByText('Loading...')).not.toBeInTheDocument();
+});
+
+// User interactions
+await user.click(screen.getByRole('button'));
+await user.type(screen.getByLabelText(/search/i), 'query');
+
+// API integration tests
+const response = await request(app)
+  .post('/api/users')
+  .send(userData)
+  .expect(201);
+```