The user-namespace regexes in the mautrix-meta-messenger and
mautrix-meta-instagram registration templates interpolated
the bridge username prefix without regex-escaping it, unlike the
neighboring homeserver domain and appservice username values. The
default prefixes are regex-safe, but a customized prefix containing
regex metacharacters would produce a wrong (too broad or invalid)
appservice user namespace.

Related to the registration regex discussion in #5096.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>