The SSRF cluster (7a6fe9bb, 48f5c425, 7ef04ae7) sealed
browser_snapshot, browser_vision, and _browser_eval against
eval-navigated private pages, but browser_get_images bypasses
_browser_eval and calls _run_browser_command("eval", ...) directly.
An eval-driven navigation to a private address followed by
browser_get_images would leak image src URLs and alt text from the
private page.
Add the same _eval_ssrf_guard_active + _current_page_private_url
recheck before returning image data, matching the pattern established
by the sibling guards.
5 new tests cover: block on private page, allow on public page, skip
for local backend, skip when private URLs allowed, no guard needed on
failed eval.
