v3 notification preferences: PUT with email_cadence but no notification_channel raises KeyError (500) #39221

PUT /api/notifications/v3/configurations/ with an e-mail cadence and no notification_channel:

{"notification_app": "discussion", "notification_type": "grouped_notification",
 "email_cadence": "Weekly"}

passes validation, then fails in the view:

KeyError: 'notification_channel'
  in _prepare_update_data
Internal Server Error: /api/notifications/v3/configurations/

With "notification_channel": "email_cadence" added, the same update succeeds (200). The account MFE always sends the channel, so this is only reachable through the API.

Cause: UserNotificationPreferenceUpdateAllSerializer.validate() deliberately lets email_cadence stand in for the channel (it only requires notification_channel when there is no email_cadence), and NotificationPreferencesViewV3._prepare_response_data infers channel = 'email_cadence' in that case. But _prepare_update_data and _log_preference_update_event both read validated_data['notification_channel'] directly, so the request 500s before the update is applied.

Expected: infer email_cadence as the channel in _prepare_update_data and _log_preference_update_event, as _prepare_response_data already does (or, if the channel is meant to be mandatory, require it in the serializer so the client gets a 400 naming the field).

Environment: Tutor main, image of 2026-09-18; code unchanged on master as of 2026-10-13. Found by the Open edX end-to-end test suite.