When the chat model cannot see images, the separate vision call's description
is appended to the user's message. Nothing marked it as machine-derived, so the
chat model read it as the user's own prose and answered the description instead
of the question.
The label goes below the `[Image: <name>]` marker line because the client
strips the block by matching that exact prefix; renaming the marker would leave
the raw description rendered in the user's bubble.
Fixes#5573
Co-authored-by: Claude <noreply@anthropic.com>
