[16.0][FIX] pms: autocheckout - replace deprecated message_post subtype kwarg

[DarioLodeiros]

Summary

Fix PmsReservation.autocheckout() which was using the deprecated subtype kwarg of message_post(), causing the cron Automatic No Checkout Reservations to fail with:

ValueError: message_post does not support subtype parameter anymore.
Please give a valid subtype_id or subtype_xmlid value instead.

The subtype kwarg was removed in Odoo 16. It now requires either subtype_id (int) or subtype_xmlid (str).

Change

 reservation.message_post(
-    subject=_("No Checkins!"), subtype="mt_comment", body=msg
+    subject=_("No Checkins!"),
+    subtype_xmlid="mail.mt_comment",
+    body=msg,
 )

Note also that the legacy "mt_comment" was implicitly assumed to be prefixed with mail.; the explicit xmlid "mail.mt_comment" keeps the same behavior.

Reproduction

Trigger the Automatic No Checkout Reservations cron (server_action #341 in default install) — any reservation reaching the autocheckout path without confirmed check-ins raises the ValueError instead of posting the chatter message and marking the reservation.

Test plan

• Cron stops failing in production after the fix
• Chatter message "No Checkins!" appears on the affected reservation

[DarioLodeiros]

/ocabot merge minor

[OCA-git-bot]

This PR looks fantastic, let's merge it!
Prepared branch 16.0-ocabot-merge-pr-399-by-DarioLodeiros-bump-minor, awaiting test results.

[OCA-git-bot]

Congratulations, your PR was merged at 53c93a0. Thanks a lot for contributing to OCA. ❤️