matrix-bot-maubot: avoid double slash in base paths when path_prefix is /

The derived `*_base_path` defaults concatenated `matrix_bot_maubot_path_prefix`
directly, producing `//v1` and `//plugin/` when users set the documented
`matrix_bot_maubot_path_prefix: /` (for serving on a dedicated subdomain),
which Traefik rejects. Apply the standard `'/' == path_prefix` guard already
used by other roles (honoroit, mautrix-discord, MAS, heisenbridge, etc.).

Reported by The Dark Wizard.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

roles/custom/matrix-bot-maubot/defaults/main.yml

@@ -23,6 +23,9 @@ matrix_bot_maubot_initial_password: ''
 
 matrix_bot_maubot_scheme: https
 matrix_bot_maubot_hostname: ''
+
+# The path at which maubot is exposed.
+# This value must either be `/` or not end with a slash (e.g. `/_matrix/maubot`).
 matrix_bot_maubot_path_prefix: /_matrix/maubot
 
 matrix_bot_maubot_container_image_self_build: false
@@ -58,9 +61,9 @@ matrix_bot_maubot_container_src_files_path: "{{ matrix_bot_maubot_base_path }}/d
 matrix_bot_maubot_customized_container_src_files_path: "{{ matrix_bot_maubot_base_path }}/customized-docker-src"
 
 matrix_bot_maubot_bot_server_public_url: "{{ matrix_bot_maubot_scheme }}://{{ matrix_bot_maubot_hostname }}"
-matrix_bot_maubot_bot_server_base_path: "{{ matrix_bot_maubot_path_prefix }}/v1"
+matrix_bot_maubot_bot_server_base_path: "{{ '' if matrix_bot_maubot_path_prefix == '/' else matrix_bot_maubot_path_prefix }}/v1"
 matrix_bot_maubot_bot_server_ui_base_path: "{{ matrix_bot_maubot_path_prefix }}"
-matrix_bot_maubot_bot_server_plugin_base_path: "{{ matrix_bot_maubot_path_prefix }}/plugin/"
+matrix_bot_maubot_bot_server_plugin_base_path: "{{ '' if matrix_bot_maubot_path_prefix == '/' else matrix_bot_maubot_path_prefix }}/plugin/"
 matrix_bot_maubot_bot_server_appservice_base_path: "{{ matrix_bot_maubot_bot_server_base_path }}"
 
 matrix_bot_maubot_proxy_management_interface: true