[FIX] mail: clean context on creation of messages with chatter

When creating records with default keys in context those are currently
propagated through the whole call chain until message creation. However
default keys are meant to be used for the record creation, not its logs
or messages.

A protection against default_child_ids already exists. However better
remove all context keys by using a clean context.

Closes odoo/odoo#93797

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>