ROLE_TURN_MARKER_RE anchored only the end of the line (\b...:\s*$), so any
outbound line that merely ended with 'user:'/'system:'/'assistant:' was
truncated — e.g. 'Please send this reply to the user:' lost its last word.
Anchor the marker to the whole line so only a standalone leaked turn marker
(its own line) is stripped; standalone-marker behavior is unchanged.
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>