fix(examples): use async sandbox in browser-use CUA

[MagMueller]

Summary

• use AsyncSandbox for the awaited browser calls in the official Browser Use CUA example
• resolve CDP inside async main while keeping is_local=True
• pass the generated left/right/middle button strings directly

Why

Browser Use is the agent in this example. The runtime failures are in the adapter: it awaits and async-iterates methods from the synchronous Sandbox client, and it treats the generated Button string type as an enum with LEFT, RIGHT, and MIDDLE members.

No Browser Use core code changes.

Checks

• Python 3.12 compile
• generated SDK probe: get_info and execute_action are coroutines; screenshot is an async generator
• mocked flow: screenshot bytes, one left click, CDP setup, Browser Use agent run, and cleanup
• Ruff diagnostics match current main; git diff --check passes
• exact one-file secret scan: zero findings

PR #181 remains separate duplicate-import, duplicate-Tools, and duplicate-main cleanup.