fix: add async_is_safe_url() to avoid blocking event loop on DNS lookup

[memosr]

is_safe_url() calls socket.getaddrinfo() which is synchronous and blocks the calling thread. It is called from web_extract_tool() and web_crawl_tool() which are both async. On a slow or unresponsive DNS server this freezes the entire asyncio event loop — blocking all gateway message handling for all users until DNS responds.

Fix extracts the hostname check into _check_hostname() and adds async_is_safe_url() which runs the blocking DNS call in a thread pool executor via loop.run_in_executor(). The original is_safe_url() is preserved for sync callers (vision_tools.py redirect hook).

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• 🐛 Bug fix (non-breaking change that fixes an issue)
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