refactor(e2e): modernize Playwright tests with latest APIs

[Copilot]

This PR refactors the e2e test suite to use modern Playwright APIs, improving test reliability and maintainability while keeping all functionality intact.

What Changed

Modernized 4 key e2e test files by replacing deprecated Playwright patterns with current best practices:

• e2e/utils/search.ts - Core search utility used across multiple test files
• e2e/fixtures/basic/index.test.ts - Main functionality tests (dark mode, navigation, styles)
• e2e/fixtures/production/index.test.ts - Production build validation
• e2e/fixtures/no-config-root/index.test.ts - Configuration-less project tests

Key Improvements

API Modernization

• Replaced deprecated page.$() with page.locator() - More robust element selection
• Eliminated page.evaluate() calls - Using direct locator methods instead
• Updated to modern assertion patterns - await expect(locator) for better error messages

Enhanced Reliability

• Improved wait strategies - Using page.waitForLoadState() and waitFor() instead of hardcoded timeouts
• Better element interactions - Using locator.fill() and locator.clear() instead of keyboard events
• Robust class validation - Using regex patterns with toHaveClass() for theme testing

Developer Experience

• Enhanced TypeScript typing - Better return types and parameter documentation
• Clearer error messages - Modern assertions provide more helpful debugging info
• Improved code readability - Consistent patterns and descriptive comments

Example of Changes

Before (deprecated pattern):

const darkModeButton = await page.$('.rspress-nav-appearance');
const html = await page.$('html');
let htmlClass = await page.evaluate(html => html?.getAttribute('class'), html);
expect(htmlClass?.includes('dark')).toBe(defaultMode !== 'dark');

After (modern pattern):

const darkModeButton = page.locator('.rspress-nav-appearance');
const htmlElement = page.locator('html');
await expect(htmlElement).toHaveClass(
  defaultMode === 'dark' ? /^(?!.*dark).*$/ : /.*dark.*/
);

Benefits

• More stable tests - Modern APIs are less prone to flakiness
• Better debugging - Clearer error messages when tests fail
• Future-proof - Uses current Playwright best practices
• Zero breaking changes - All test functionality preserved

All changes maintain 100% backward compatibility while bringing the test suite up to current Playwright standards (v1.55.0).

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