_humanize_user_mentions rewrites <@UID> to @DisplayName by passing the
resolved name as re.sub's replacement, where re parses it as a template.
A display name is arbitrary user-set text, so the escapes in it are the
user's characters, not regex syntax:
dev\ops -> re.error: bad escape \o
a\1b -> re.error: invalid group reference 1
\g<0> -> expands to the whole match, silently putting the opaque
<@UID> back — the token this method exists to remove
The trigger-text call site sits in _handle_slack_message outside any try,
and both Bolt event handlers await it bare, so the raise takes the whole
inbound message down: every message mentioning that person is dropped.
Pass the replacement as a function instead — re does no template parsing
on the return value, so the name lands verbatim. Same shape the Matrix
adapter already uses for its outbound mention rewrite.
7c02bfce89