Restored ToJSON encoding (sans Integration) for Post APIs (#19279)

* Restored ToJSON encoding sans Integration for Post APIs

* PR feedback + GetPost test

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/channel.go

@@ -682,7 +682,7 @@ func getPinnedPosts(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 
 	w.Header().Set(model.HeaderEtagServer, clientPostList.Etag())
-	if err := json.NewEncoder(w).Encode(clientPostList); err != nil {
+	if err := clientPostList.EncodeJSON(w); err != nil {
 		mlog.Warn("Error while writing response", mlog.Err(err))
 	}
 }