_maybe_wrap_untrusted() only wrapped str-typed tool outputs. When a
high-risk tool (web_extract, browser_*) returns a multimodal content
list ([{type:text},{type:image_url}]) — which _tool_result_content_for
_active_model() produces by unwrapping the _multimodal envelope for
vision-capable providers — the text part reached the model completely
unguarded. An attacker page that ships one image bypassed the entire
untrusted-data wrapper.
Extend the wrapper to handle list content: each {type:text} part is run
through the same string-wrapping path (min-char threshold, delimiter
neutralization, one well-formed block), image/video parts pass through
untouched so the list stays valid for vision adapters. Recursing into
the existing string branch means the list path inherits the delimiter
defang and the no-forgeable-fast-path hardening from #56172 for free.
The outer list is rebuilt (not returned by identity), so callers compare
by value.
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