MM-17339: fix missing post thread (#11729)

* leverage testify for TestGetPostsForChannelAroundLastUnread

* introduce assertPostList helper

* unit test MM-17339

* fix MM-17339

app/post.go

@@ -771,27 +771,33 @@ func (a *App) GetPostsForChannelAroundLastUnread(channelId, userId string, limit
 		return model.NewPostList(), nil
 	}
 
-	lastUnreadPost, err := a.GetPostAfterTime(channelId, member.LastViewedAt)
+	lastUnreadPostId, err := a.GetPostIdAfterTime(channelId, member.LastViewedAt)
 	if err != nil {
 		return nil, err
-	} else if lastUnreadPost == nil {
+	} else if lastUnreadPostId == "" {
 		return model.NewPostList(), nil
 	}
 
-	var postList *model.PostList
-	if postList, err = a.GetPostsBeforePost(channelId, lastUnreadPost.Id, PAGE_DEFAULT, limitBefore); err != nil {
+	postList, err := a.GetPostThread(lastUnreadPostId)
+	if err != nil {
 		return nil, err
 	}
+	// Reset order to only include the last unread post: if the thread appears in the centre
+	// channel organically, those replies will be added below.
+	postList.Order = []string{lastUnreadPostId}
 
-	if postListAfter, err := a.GetPostsAfterPost(channelId, lastUnreadPost.Id, PAGE_DEFAULT, limitAfter-1); err != nil {
+	if postListBefore, err := a.GetPostsBeforePost(channelId, lastUnreadPostId, PAGE_DEFAULT, limitBefore); err != nil {
+		return nil, err
+	} else if postListBefore != nil {
+		postList.Extend(postListBefore)
+	}
+
+	if postListAfter, err := a.GetPostsAfterPost(channelId, lastUnreadPostId, PAGE_DEFAULT, limitAfter-1); err != nil {
 		return nil, err
 	} else if postListAfter != nil {
 		postList.Extend(postListAfter)
 	}
 
-	postList.AddPost(lastUnreadPost)
-	postList.AddOrder(lastUnreadPost.Id)
-
 	postList.SortByCreateAt()
 	return postList, nil
 }