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/store.go

@@ -390,7 +390,7 @@ type PostStore interface {
 	GetMaxPostSize() int
 	GetParentsForExportAfter(limit int, afterID string, includeArchivedChannels bool) ([]*model.PostForExport, error)
 	GetRepliesForExport(parentID string) ([]*model.ReplyForExport, error)
-	GetDirectPostParentsForExportAfter(limit int, afterID string) ([]*model.DirectPostForExport, error)
+	GetDirectPostParentsForExportAfter(limit int, afterID string, includeArchivedChannels bool) ([]*model.DirectPostForExport, error)
 	SearchPostsForUser(rctx request.CTX, paramsList []*model.SearchParams, userID, teamID string, page, perPage int) (*model.PostSearchResults, error)
 	GetOldestEntityCreationTime() (int64, error)
 	HasAutoResponsePostByUserSince(options model.GetPostsSinceOptions, userId string) (bool, error)