[IMP] mail: simplify parent and parent internal computation in routing

PURPOSE

Clean posting process and improve mail.message definition and comprehension.

SPECIFICATIONS

In this commit we use the is_internal flag to correctly propagate it from
mail gateway and clean some parent check. In this commit some code is cleaned.
No functional change should occur with this commit.

LINKS

Task ID 2071556
PR #38692

addons/mail/models/mail_thread.py

@@ -1437,18 +1437,15 @@ class MailThread(models.AbstractModel):
                 stored_date = datetime.datetime.now()
             msg_dict['date'] = stored_date.strftime(tools.DEFAULT_SERVER_DATETIME_FORMAT)
 
+        parent_ids = False
         if msg_dict['in_reply_to']:
             parent_ids = self.env['mail.message'].search([('message_id', '=', msg_dict['in_reply_to'])], limit=1)
-            if parent_ids:
-                msg_dict['parent_id'] = parent_ids.id
-                msg_dict['is_internal'] = parent_ids.subtype_id and parent_ids.subtype_id.internal or False
-
-        if msg_dict['references'] and 'parent_id' not in msg_dict:
+        if msg_dict['references'] and not parent_ids:
             references_msg_id_list = tools.mail_header_msgid_re.findall(msg_dict['references'])
             parent_ids = self.env['mail.message'].search([('message_id', 'in', [x.strip() for x in references_msg_id_list])], limit=1)
-            if parent_ids:
-                msg_dict['parent_id'] = parent_ids.id
-                msg_dict['is_internal'] = parent_ids.subtype_id and parent_ids.subtype_id.internal or False
+        if parent_ids:
+            msg_dict['parent_id'] = parent_ids.id
+            msg_dict['is_internal'] = parent_ids.subtype_id and parent_ids.subtype_id.internal or False
 
         msg_dict.update(self._message_parse_extract_payload(message, save_original=save_original))
         msg_dict.update(self._message_parse_extract_bounce(message, msg_dict))