Stop testing on error pages

[DenysKuchma]

Summary

Stops exploration/test execution when the current page is detected as an HTTP error page. #54

Changes

• Capture main document HTTP status from Playwright navigation responses
• Store httpStatus in page state/action results.
• Detect error pages by HTTP status before falling back to existing title/heading checks
• Stop explore before planning when the current page is an error page
• Stop test before AI execution when the start/current page is an error page
• Clear stale testing activity after early abort.
• Add unit coverage for error-page detection, explore guard, and tester abort flow

[DavertMik]

minor ideas