fix: map empty attribute to _id in Mongo::getInternalKeyForAttribute

Query::orderAsc() with no attribute produces an empty string. SQL
adapters treat this as a no-op ORDER BY on the auto-increment key,
but Mongo built an invalid sort on '' causing queries to hang.

Map '' -> '_id' alongside '$sequence' so empty orderAsc() sorts by
the MongoDB sequence key, matching SQL adapter behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: abnegate

src/Database/Adapter/Mongo.php

@@ -1978,7 +1978,7 @@ protected function getInternalKeyForAttribute(string $attribute): string
     {
         return match ($attribute) {
             '$id' => '_uid',
-            '$sequence' => '_id',
+            '', '$sequence' => '_id',
             '$collection' => '_collection',
             '$tenant' => '_tenant',
             '$createdAt' => '_createdAt',