fix: remove notification failure policy log record when prevent posting is set (#5260)

# What this PR does
Changes UserNotificationPolicyLogRecord to success when
slack_prevent_posting is set as the user has already been notified in
slack or another method in their personal notification preferences.
These entries have also been filtered out of the alert group history
timeline as they were causing confusion to users thinking notifications
failed when in fact they had already been sent.

## Which issue(s) this PR closes

https://github.com/grafana/support-escalations/issues/13236

## Checklist

- [x] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] Added the relevant release notes label (see labels prefixed w/
`release:`). These labels dictate how your PR will
    show up in the autogenerated release notes.

engine/apps/alerts/incident_log_builder/incident_log_builder.py

@@ -152,6 +152,15 @@ class IncidentLogBuilder:
                     Q(type=UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_TRIGGERED)
                     & Q(notification_policy__step=UserNotificationPolicy.Step.WAIT)
                 )
+                # Exclude SUCCESS + ERROR_NOTIFICATION_POSTING_TO_SLACK_IS_DISABLED, these cause confusions as the user
+                # has already been notified by another path so this step should not be displayed, although it is kept
+                # for auditing.
+                | Q(
+                    Q(type=UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_SUCCESS)
+                    & Q(
+                        notification_error_code=UserNotificationPolicyLogRecord.ERROR_NOTIFICATION_POSTING_TO_SLACK_IS_DISABLED
+                    )
+                )
             )
             .select_related("author")
             .distinct()

engine/apps/alerts/tasks/notify_user.py

@@ -528,7 +528,7 @@ def perform_notification(log_record_pk, use_default_notification_policy_fallback
                 )
                 UserNotificationPolicyLogRecord(
                     author=user,
-                    type=UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_FAILED,
+                    type=UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_SUCCESS,
                     notification_policy=notification_policy,
                     reason="Prevented from posting in Slack",
                     alert_group=alert_group,

engine/apps/alerts/tests/test_notify_user.py

@@ -315,7 +315,7 @@ def test_perform_notification_slack_prevent_posting(
 
     mocked_send_slack_notification.assert_not_called()
     last_log_record = UserNotificationPolicyLogRecord.objects.last()
-    assert last_log_record.type == UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_FAILED
+    assert last_log_record.type == UserNotificationPolicyLogRecord.TYPE_PERSONAL_NOTIFICATION_SUCCESS
     assert last_log_record.reason == "Prevented from posting in Slack"
     assert (
         last_log_record.notification_error_code