Follow-up to #56874, which added the Camofox private-page SSRF guard
(_camofox_current_page_private_url) but wired it only into the Camofox
eval path (_camofox_eval). The other Camofox content-read tools —
camofox_snapshot, camofox_get_images, and camofox_vision — still read the
current page's accessibility tree / images / screenshot without the
guard, so on a non-local Camofox backend they can return the content of
an intranet or cloud-metadata page (e.g. 169.254.169.254) that the
terminal itself can't reach.
Apply the same guard, gated on _eval_ssrf_guard_active (non-local
backend, not a local sidecar, allow_private_urls unset) and fail-open on
probe failure, matching the eval-path guard and the main-browser
snapshot/vision guards. camofox_back is intentionally not changed: its
target is unknown until navigation completes, and the subsequent content
read is already guarded.
Adds regression tests covering the three read tools blocking on a private
page, the public-page pass-through, and the guard-inactive no-probe path.
