Purpose is to test behavior when having multiple references in mail headers.
We have to check that parent_id is correctly taken (as it impacts internal
flag), and that flattening is performance when posting the message (always
attaching to the thread first message being the default behavior).
Note that tests highlighted that flattening is currently a bit broken as it
does not go up until the first ancestor, which is the expected behavior for
business documents having ``_mail_flat_thread``. Next commit will fix that.
Task-2822652 (Mail: fix parent message fetch / flattening)
Task-2643114 (Mail: Message-Id in references to ease thread formation)
X-original-commit: f7d4f1d29213c71e76564c485db2bbecf0bbf37c
Part-of: odoo/odoo#89328
Thibault Delavallée
4193b1ee64