MM-56579: Include deactivated users from DM posts (#25985)

There were 2 separate bugs here:
1. We were not including deactivated users while fetching posts from
DM channels.
2. We were not respecting the includeArchivedChannels flag while
fetching DM channels.

We fix both of these issues here.

```release-note
Include posts from deactivated users in DM channel export. Also
respect the --include-archived-channels flag for DM channels.
```

https://mattermost.atlassian.net/browse/MM-56579

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/timerlayer/timerlayer.go

@@ -5610,10 +5610,10 @@ func (s *TimerLayerPostStore) Get(ctx context.Context, id string, opts model.Get
 	return result, err
 }
 
-func (s *TimerLayerPostStore) GetDirectPostParentsForExportAfter(limit int, afterID string) ([]*model.DirectPostForExport, error) {
+func (s *TimerLayerPostStore) GetDirectPostParentsForExportAfter(limit int, afterID string, includeArchivedChannels bool) ([]*model.DirectPostForExport, error) {
 	start := time.Now()
 
-	result, err := s.PostStore.GetDirectPostParentsForExportAfter(limit, afterID)
+	result, err := s.PostStore.GetDirectPostParentsForExportAfter(limit, afterID, includeArchivedChannels)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {