MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/platform/services/sharedchannel/sync_send_remote.go

@@ -270,7 +270,7 @@ func (scs *Service) fetchPostsForSync(sd *syncData) error {
 		return fmt.Errorf("could not fetch new posts for sync: %w", err)
 	}
 	count := len(posts)
-	sd.posts = appendPosts(sd.posts, posts, scs.server.GetStore().Post(), cursor.LastPostCreateAt)
+	sd.posts = appendPosts(sd.posts, posts, scs.server.GetStore().Post(), cursor.LastPostCreateAt, scs.server.Log())
 
 	cache := postsSliceToMap(posts)
 
@@ -284,7 +284,7 @@ func (scs *Service) fetchPostsForSync(sd *syncData) error {
 		}
 		posts = reducePostsSliceInCache(posts, cache)
 		count += len(posts)
-		sd.posts = appendPosts(sd.posts, posts, scs.server.GetStore().Post(), cursor.LastPostUpdateAt)
+		sd.posts = appendPosts(sd.posts, posts, scs.server.GetStore().Post(), cursor.LastPostUpdateAt, scs.server.Log())
 	}
 
 	sd.resultNextCursor = nextCursor
@@ -293,7 +293,7 @@ func (scs *Service) fetchPostsForSync(sd *syncData) error {
 	return nil
 }
 
-func appendPosts(dest []*model.Post, posts []*model.Post, postStore store.PostStore, timestamp int64) []*model.Post {
+func appendPosts(dest []*model.Post, posts []*model.Post, postStore store.PostStore, timestamp int64, logger mlog.LoggerIFace) []*model.Post {
 	// Append the posts individually, checking for root posts that might appear later in the list.
 	// This is due to the UpdateAt collision handling algorithm where the order of posts is not based
 	// on UpdateAt or CreateAt when the posts have the same UpdateAt value. Here we are guarding
@@ -302,7 +302,7 @@ func appendPosts(dest []*model.Post, posts []*model.Post, postStore store.PostSt
 	// happens during load testing or bulk imports.
 	for _, p := range posts {
 		if p.RootId != "" {
-			root, err := postStore.GetSingle(p.RootId, true)
+			root, err := postStore.GetSingle(request.EmptyContext(logger), p.RootId, true)
 			if err == nil {
 				if (root.CreateAt >= timestamp || root.UpdateAt >= timestamp) && !containsPost(dest, root) {
 					dest = append(dest, root)