Add mautrix/appservice slack bridge bot username checks

roles/matrix-bridge-mautrix-slack/tasks/validate_config.yml

@@ -8,3 +8,12 @@
   with_items:
     - "matrix_mautrix_slack_appservice_token"
     - "matrix_mautrix_slack_homeserver_token"
+
+- name: Fail if appservice-slack and mautrix-slack have conflicting bot usernames
+  when: matrix_appservice_slack_enabled | default(False) | bool and matrix_mautrix_slack_appservice_bot_username == matrix_appservice_slack_bot_name | default ('')
+  ansible.builtin.fail:
+    msg: |
+      The appservice-slack and mautrix-slack components are both enabled and use the same bot username ({{ matrix_mautrix_slack_appservice_bot_username }}), as per their default configuration, which causes a conflcit.
+      To resolve the conflict, make one of these components use a different username.
+      Consider either changing `matrix_mautrix_slack_appservice_bot_username` (the bot username for the mautrix-slack component) or `matrix_appservice_slack_bot_name` (the bot username for the appservice-slack component).
+      We recommend that you change the username for the newly-added (and yet unused) component.