Both chat pages capped the "conversation finished" notification body with
String#length / substring, which count UTF-16 code units: a cut inside an
astral character's surrogate pair rendered U+FFFD, and a body that fits 200
code points was shortened early.
Extract notificationBody() so both pages share the code-point bound and the
"Conversation finished" fallback, and add boundary regressions.
Co-authored-by: 一片一片 <192845873+gdut4140@users.noreply.github.com>
