Add integration tests for StackOverflow page download with markdown and image support

[konard]

Summary

This PR adds comprehensive integration tests to verify that both Puppeteer and Playwright browser engines can successfully download and process a real-world StackOverflow page, including:

• Converting the page HTML to markdown format
• Capturing screenshots of the page as PNG images

Changes

New Tests (tests/integration/browser-engines.test.js)

Added test suite "StackOverflow Page Download" with tests for both engines:

Puppeteer Engine:

• ✅ Can download StackOverflow page and convert to markdown
• ✅ Can take a screenshot of StackOverflow page

Playwright Engine:

• ✅ Can download StackOverflow page and convert to markdown
• ✅ Can take a screenshot of StackOverflow page

The tests verify:

1. Page content is successfully retrieved (HTML > 1000 chars)
2. Markdown conversion produces valid output (> 100 chars)
3. Markdown contains expected content ("How do I undo the most recent local commits in Git")
4. Screenshots are valid PNG files with correct signature bytes
5. Both engines work with the specific URL: https://stackoverflow.com/questions/927358/how-do-i-undo-the-most-recent-local-commits-in-git

Configuration Updates

• jest.config.mjs: Added **/tests/integration/**/*.test.js to testMatch pattern to enable integration tests

Bug Fixes

• src/browser.js: Fixed Playwright adapter's setUserAgent method. Playwright doesn't have a page.setUserAgent() method, so implemented it using route interception to modify request headers

Performance Tuning

• Increased navigation timeout to 60000ms (60s) for StackOverflow tests to handle slower page loads
• Increased test timeout to 90000ms (90s) to ensure tests have enough time to complete

Test Results

All 11 tests pass successfully:

Test Suites: 1 passed, 1 total
Tests:       11 passed, 11 total
Time:        116.733 s

Fixes

Fixes #11

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[konard]

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