yuanbao_media.download_url() fetched model-supplied (outbound) and inbound
image/file URLs server-side via httpx with follow_redirects=True and no
SSRF check. A model response containing <img src="http://169.254.169.254/...">
routed through ImageUrlHandler -> download_url and would fetch cloud-metadata
endpoints; same for inbound media.
Add an is_safe_url() pre-flight plus an async redirect event-hook that
re-validates every 30x target, matching the cache_image_from_url() guard in
gateway/platforms/base.py. The other gateway adapters already guard their
URL-fetch paths; this was the remaining unguarded one.
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