fix: replace blocking synchronous I/O with async httpx in CDP browser startup

[rahulrao85]

Summary

The CDP browser startup helpers used blocking \urllib.request\ and \ ime.sleep()\ inside async methods, blocking the asyncio event loop for up to 10 seconds. This is particularly problematic because the agent loop cannot make progress or be cancelled during CDP polling.

Changes

• Added async versions of _is_local_cdp_available, _local_cdp_page_targets, and _ensure_local_cdp_page_target\ using \httpx.AsyncClient\ (httpx is already a project dependency)
• Converted _ensure_local_cdp_browser\ to async with \�syncio.sleep()\ instead of \ ime.sleep()\
• Updated _prepare_async\ to await the new async methods
• Removed unused \urllib.request\ imports (\ProxyHandler, \Request, \�uild_opener)

Fixes: #56

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