[FIX] mail: allow message edit when livechat is installed

Livechat override of `_message_update_content` accesses `self` channel
but it was not called with the guest env, leading to access error.

Part-of: odoo/odoo#128290

addons/mail/controllers/thread.py

@@ -97,6 +97,6 @@ class ThreadController(http.Controller):
             raise NotFound()
         if not message_sudo.model or not message_sudo.res_id:
             raise NotFound()
-        request.env[message_sudo.model].browse([message_sudo.res_id])._message_update_content(
+        guest.env[message_sudo.model].browse([message_sudo.res_id])._message_update_content(
             message_sudo, body, attachment_ids=attachment_ids, partner_ids=partner_ids
         )