[FIX] mail: support multi email in '_mail_find_partner_from_emails'

PURPOSE

Be defensive when dealing with email fields, notably when having multi-emails
or email field containing an already-formatted email.

SPECIFICATIONS

Tool method '_mail_find_partner_from_emails' that searches for partners based
on email is improved to support multi-emails input. Instead of skipping
multi-email (current behavior of 'email_normalize') we now consider first
found email in the input.

It is used notably to match incoming email / partners or suggest recipients
in Chatter.

Task-2612945 (Mail: Defensive email formatting)

Part-of: odoo/odoo#74474