[FIX] website_livechat: missing decorator on message_post

Incorrect override of message_post.
Without this decorator, message_post returns a string `mail.message(ID,)` via
RPC. Discuss needs the message ID to scroll chat window

closes odoo/odoo#57123

X-original-commit: 3939d3aa3b60e0f606a9f7defd695417f7b3f383
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>

addons/test_mail/tests/test_message_post.py

@@ -10,6 +10,7 @@ from odoo.addons.test_mail.models.test_mail_models import MailTestSimple
 from odoo.addons.test_mail.tests.common import TestMailCommon, TestRecipients
 from odoo.exceptions import AccessError
 from odoo.tools import mute_logger, formataddr
+from odoo.api import call_kw
 
 
 class TestMessagePost(TestMailCommon, TestRecipients):
@@ -294,3 +295,13 @@ class TestMessagePost(TestMailCommon, TestRecipients):
                 subject='About %s' % test_record.name,
                 body_content=test_record.name,
                 attachments=[('first.txt', b'My first attachment', 'text/plain'), ('second.txt', b'My second attachment', 'text/plain')])
+
+    # This method should be run inside a post_install class to ensure that all
+    # message_post overrides are tested.
+    def test_message_post_return(self):
+        test_channel = self.env['mail.channel'].create({
+            'name': 'Test',
+        })
+        # Use call_kw as shortcut to simulate a RPC call.
+        messageId = call_kw(self.env['mail.channel'], 'message_post', [test_channel.id], {'body': 'test'})
+        self.assertTrue(isinstance(messageId, int))

addons/website_livechat/models/mail_channel.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
-from odoo import fields, models, _
+from odoo import api, fields, models, _
 
 
 class MailChannel(models.Model):
@@ -62,6 +62,7 @@ class MailChannel(models.Model):
 
         return message
 
+    @api.returns('mail.message', lambda value: value.id)
     def message_post(self, **kwargs):
         """Override to mark the visitor as still connected.
         If the message sent is not from the operator (so if it's the visitor or