[FIX] im_livechat: make /history command work for public user

Have the public user (not logged in -- user B) having a live chat with any logged in user (user A)
Having in mind that:
- (A) issues /history in his chatter to see where (B) has gone on the website
- The bus sends a kind of a "pull cookie request" on each clients
- The browser of (B) retrieves the cookie info and calls the relevant route
- That route sends the transient message for (A)

Before this commit:
The command did not work because the user of the route we end up on is the public one
hence, the search for the channel would not yield anything

After this commit:
The command does work as expected

OPW 1818732
closes #23485

addons/im_livechat/controllers/main.py

@@ -110,7 +110,8 @@ class LivechatController(http.Controller):
 
     @http.route('/im_livechat/history', type="json", auth="public")
     def history_pages(self, pid, channel_uuid, page_history=None):
-        channel = request.env['mail.channel'].search([('uuid', '=', channel_uuid)])
+        partner_ids = (pid, request.env.user.partner_id.id)
+        channel = request.env['mail.channel'].sudo().search([('uuid', '=', channel_uuid), ('channel_partner_ids', 'in', partner_ids)])
         if channel:
             channel._send_history_message(pid, page_history)
         return True