Escape mautrix-meta username prefix in registration regexes

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>

roles/custom/matrix-bridge-mautrix-meta-instagram/defaults/main.yml

@@ -316,7 +316,7 @@ matrix_mautrix_meta_instagram_registration_yaml: |
   namespaces:
     users:
     - exclusive: true
-      regex: '^@{{ matrix_mautrix_meta_instagram_bridge_username_prefix }}.+:{{ matrix_mautrix_meta_instagram_homeserver_domain | regex_escape }}$'
+      regex: '^@{{ matrix_mautrix_meta_instagram_bridge_username_prefix | regex_escape }}.+:{{ matrix_mautrix_meta_instagram_homeserver_domain | regex_escape }}$'
     - exclusive: true
       regex: '^@{{ matrix_mautrix_meta_instagram_appservice_username | regex_escape }}:{{ matrix_mautrix_meta_instagram_homeserver_domain | regex_escape }}$'
   url: {{ matrix_mautrix_meta_instagram_appservice_address }}

roles/custom/matrix-bridge-mautrix-meta-messenger/defaults/main.yml

@@ -316,7 +316,7 @@ matrix_mautrix_meta_messenger_registration_yaml: |
   namespaces:
     users:
     - exclusive: true
-      regex: '^@{{ matrix_mautrix_meta_messenger_bridge_username_prefix }}.+:{{ matrix_mautrix_meta_messenger_homeserver_domain | regex_escape }}$'
+      regex: '^@{{ matrix_mautrix_meta_messenger_bridge_username_prefix | regex_escape }}.+:{{ matrix_mautrix_meta_messenger_homeserver_domain | regex_escape }}$'
     - exclusive: true
       regex: '^@{{ matrix_mautrix_meta_messenger_appservice_username | regex_escape }}:{{ matrix_mautrix_meta_messenger_homeserver_domain | regex_escape }}$'
   url: {{ matrix_mautrix_meta_messenger_appservice_address }}