fix(browser-use-demo): declare httpx runtime dependency

[00200200]

Summary

• declare httpx, which browser_use_demo.loop imports at runtime
• keep fresh installations from failing with ModuleNotFoundError

Verification

• installed browser_use_demo/requirements.txt in a clean virtual environment
• imported browser_use_demo.loop successfully
• pytest tests -m "not integration": 69 passed

This is a small dependency fix discovered while reviewing the browser-use demo installation path.