[FIX] mail: set message_main_attachment_id to copy=False

Create a record.
Add an attachment set as message_main_attachment_id (e.g. through log note).
Duplicate that record.
The new record has the message_main_attachment_id of the old record,
while it has no attachments.

Since attachments security is linked to the record id, we shall not copy them.

opw 1950403

closes odoo/odoo#31846

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/mail/models/mail_thread.py

@@ -110,7 +110,7 @@ class MailThread(models.AbstractModel):
         'Number of error', compute='_compute_message_has_error',
         help="Number of messages with delivery error")
     message_attachment_count = fields.Integer('Attachment Count', compute='_compute_message_attachment_count')
-    message_main_attachment_id = fields.Many2one(string="Main Attachment", comodel_name='ir.attachment', index=True)
+    message_main_attachment_id = fields.Many2one(string="Main Attachment", comodel_name='ir.attachment', index=True, copy=False)
 
     @api.one
     @api.depends('message_follower_ids')