briefReason() measured and sliced UTF-16 code units, so a cut inside an
emoji's surrogate pair left a lone high surrogate that the outcome notice
renders as U+FFFD. Measuring code points also stops shortening a summary
that fits the documented budget.
Add a regression test for both the fits-exactly and truncating cases.
Co-authored-by: 一片一片 <192845873+gdut4140@users.noreply.github.com>