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/channels/store/searchlayer/file_info_layer.go

@@ -23,7 +23,7 @@ func (s SearchFileInfoStore) indexFile(rctx request.CTX, file *model.FileInfo) {
 				if file.PostId == "" {
 					return
 				}
-				post, postErr := s.rootStore.Post().GetSingle(file.PostId, false)
+				post, postErr := s.rootStore.Post().GetSingle(rctx, file.PostId, false)
 				if postErr != nil {
 					rctx.Logger().Error("Couldn't get post for file for SearchEngine indexing.", mlog.String("post_id", file.PostId), mlog.String("search_engine", engineCopy.GetName()), mlog.String("file_info_id", file.Id), mlog.Err(postErr))
 					return