Add comprehensive testing documentation for applications using DB UX components

[Copilot]

This PR adds comprehensive documentation to help developers test applications that use DB UX Design System components. The new guide covers all major supported frameworks and addresses common testing challenges.

What's Added

New Documentation: docs/how-to-test-with-db-ux-components.md

This comprehensive guide provides framework-specific testing recommendations for:

• React - Setup with React Testing Library, Jest, and Playwright
• Vue - Testing with Vue Test Utils and Vitest
• Angular - Using Angular Testing Utilities and component fixtures
• Stencil/Web Components - Custom element registration and testing

Key Features

Complete Setup Instructions

• Package installation commands for each framework
• CSS styling configuration for test environments
• Global test setup examples with proper imports

Practical Testing Examples

• Component interaction testing (button clicks, form inputs)
• Mocking strategies for unit testing
• TypeScript integration and proper component typing
• Event handling across different frameworks

Common Testing Scenarios

• Form submission and validation testing
• Navigation component interactions
• Accessibility testing with axe-core integration
• Component state management testing

Comprehensive Troubleshooting

• CSS import issues and solutions
• Icon loading problems and mocking strategies
• Custom element registration for Web Components
• TypeScript type resolution
• Framework-specific event handling differences

Example Usage

// React Testing Example
import { render, screen, fireEvent } from '@testing-library/react';
import { DBButton } from '@db-ux/react-core-components';

test('should handle button click', () => {
  const handleClick = jest.fn();
  render(<DBButton onClick={handleClick}>Click me</DBButton>);
  
  fireEvent.click(screen.getByRole('button'));
  expect(handleClick).toHaveBeenCalled();
});

The documentation includes similar examples for Vue, Angular, and Web Components, covering all the scenarios developers commonly encounter when testing applications that use DB UX components.

This addresses the need for clear guidance on mocking strategies, component typing, interaction testing, and framework-specific best practices that were previously undocumented.

Fixes #3483.

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