fix(cleanup): remove XPI temp files from /tmp on shutdown

[vringar]

Summary

• Track temporary XPI files created by geckodriver during install_addon() and clean them up on browser shutdown/restart
• Move tempdir cleanup outside the success block so it runs even on failed browser launches
• Use extend() instead of = when collecting tmp_files to avoid leaking files from retried spawn attempts

Closes #1090

Changes

• openwpm/deploy_browsers/deploy_firefox.py: Detect new XPI files via glob before/after install_addon(), report them via status queue
• openwpm/browser_manager.py: Track tmp_files on BrowserManagerHandle, clean up in shutdown_browser() and restart_browser_manager()

Test plan

• Verify XPI files are cleaned up after browser shutdown
• Verify temp profile directories are cleaned up even on failed launches
• Run full test suite to check for regressions

[vringar]

Closing in favor of #1104 — the racy tracking approach doesn't align with OpenWPM's robustness-first philosophy.