Add async_is_safe_url() wrapping is_safe_url via asyncio.to_thread, and route
all async SSRF call sites through it: web_extract_tool, the vision/video
preflight checks, and both download redirect guards. socket.getaddrinfo blocks;
calling it inline from async tool paths froze the event loop for the duration of
DNS resolution.
vision_tools: split _validate_image_url into _image_url_shape_ok (no DNS) +
sync _validate_image_url (for sync callers/tests) + async _validate_image_url_async.
Widened beyond the original PR #3691 to sibling async sites that also blocked
the loop (second redirect guard, video preflight).
Salvage of #3691 by @Kewe63 — surgically re-applied onto current main because
the original branch was too stale to cherry-pick cleanly (would have reverted
the web_crawl_tool refactor).
Co-authored-by: Kewe63 <kewe.3217@gmail.com>
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