[PLUGIN][ActivityPub][Inbox] Accept Follow Activity

Improve how Core Activity is handled in general

plugins/ActivityPub/Util/Model/Note.php

@@ -308,7 +308,7 @@ class Note extends Model
     public static function toJson(mixed $object, ?int $options = null): string
     {
         if ($object::class !== GSNote::class) {
-            throw new InvalidArgumentException('First argument type is Note');
+            throw new InvalidArgumentException('First argument type must be a Note.');
         }
 
         $attr = [