Inside httpx AsyncClient response event hooks, response.next_request is
often None even for a genuine redirect, so guards keyed on
`if response.is_redirect and response.next_request` silently never fire.
A public URL that 302s to http://169.254.169.254/ was followed anyway,
defeating the pre-flight is_safe_url() check.
Resolve the redirect target from the Location header (via urljoin, so
relative Locations work too), falling back to next_request only when no
Location is present. Extracted as tools.url_safety.redirect_target_from_response
and wired into every SSRF redirect guard:
- gateway/platforms/base.py (shared image + audio download for all platforms)
- tools/vision_tools.py (two download hooks)
- plugins/platforms/slack/adapter.py
Original fix by @zapabob (PR #35940), which targeted the since-refactored
gateway/platforms/slack.py; reconstructed onto the current shared sites and
widened to the whole bug class.
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