`image_search` set both `image_url` and `thumbnail_url` to the DDGS result's
`thumbnail` field, so the tool never returned the full-resolution image even
though its docstring/usage_hint promise reference-quality images (and a result
with no `thumbnail` returned an empty `image_url`). DDGS `.images()` exposes the
full-res source under the separate `image` key — read that for `image_url`,
matching the serper/brave providers which keep image vs thumbnail distinct.
Adds a regression test.
