fix(optimize): drop phantom detach offers from non-mount hdiutil lines

opt_diag_parse_image_mount_pairs treated any line in hdiutil info with a
whitespace-prefixed absolute path as a mount point. That included
image-alias, icon-path, and similar metadata fields whose values are
just paths, not real mounts. The most visible symptom (#960) was
seeing the same disk image offered for detach twice, once at the
genuine /Volumes/<name> mount and again with the .dmg's own path as
the apparent mount point.

Tighten extract_mount to only look at /dev/disk* lines, which are the
only entries in hdiutil info that legitimately carry a mount column.
Add two bats cases covering the report's exact pattern and a
multi-block input to guard against regressions.