[FIX] mail: make mail_to computation of mailing traces deterministic

When having several possible emails to contact the first one is saved on the
trace model. Indeed a trace is normally linked to a unique recipient but
in case of multi-emails in an email field, several emails are available.

This commit ensure computation keeps the original mail_to when making emails
adresses uniques before sending mails and creating traces.

Followup of odoo/odoo@213d5d396c811d8e07ce928c0d31a7f47756a644

Task-2612945 (Mail: Defensive email formatting)

X-original-commit: odoo/odoo@1ef74dc6a730b4bc2936bf0abf02f96fdd97cf8e
Part-of: odoo/odoo#134188

addons/mail/wizard/mail_compose_message.py

@@ -478,7 +478,10 @@ class MailComposer(models.TransientModel):
                 for recipient_command in mail_values.get('recipient_ids') or []
                 if recipient_command[1]
             ]
-            mail_to = list(set(mail_to))
+            # uniquify, keep ordering
+            seen = set()
+            mail_to = [email for email in mail_to if email not in seen and not seen.add(email)]
+
             recipients_info[record_id] = {
                 'mail_to': mail_to,
                 'mail_to_normalized': [