[FIX] mail: remove microseconds when parsing vague datetime

Mail holds an helper function to convert datetime from a char input into a
datetime value. It is used notably when having to store a datetime value
(e.g. on a mail_mail) from a string generated by templates with expression
like ``{{ datetime.datetime.now() + datetime.timedelta(days=2)}}``.

In this commit we explicitly remove microseconds, as otherwise we end up with
something that the web client cannot parse, not speaking of even going into
server side storage with this value not matching the expected format.

Task-2710804 (Mail: Clean MailThread Posting API)

Part-of: odoo/odoo#99482

addons/mail/models/mail_mail.py

@@ -309,6 +309,7 @@ class MailMail(models.Model):
             except (ValueError, TypeError):
                 parsed_datetime = False
         if parsed_datetime:
+            parsed_datetime = parsed_datetime.replace(microsecond=0)
             if not parsed_datetime.tzinfo:
                 parsed_datetime = pytz.utc.localize(parsed_datetime)
             else: