push_notification: Use durable=True for the outermost transaction.

This commit updates the `transaction.atomic` context manager
used to take a lock on Message row in `handle_push_notification`
to use `durable=True` instead of `savepoint=False`. It helps to
avoid introducing an outer transaction by mistake - resulting in
locking row for a longer duration.

Signed-off-by: Prakhar Pratyush <prakhar@zulip.com>

zerver/lib/push_notifications.py

@@ -1639,7 +1639,7 @@ def handle_push_notification(user_profile_id: int, missed_message: dict[str, Any
         # BUG: Investigate why it's possible to get here.
         return  # nocoverage
 
-    with transaction.atomic(savepoint=False):
+    with transaction.atomic(durable=True):
         try:
             (message, user_message) = access_message_and_usermessage(
                 user_profile,