fix: skip schema index validation during shared-tables migration

Match the attribute validator's behavior: when isMigrating is true in
shared-tables mode, skip the schema index check so indexes are always
created fresh during migration rather than being matched against or
dropping existing physical indexes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: abnegate

src/Database/Database.php

@@ -4576,7 +4576,8 @@ public function createIndex(string $collection, string $id, string $type, array
         $created = false;
         $existsInSchema = false;
 
-        if ($this->adapter->getSupportForSchemaIndexes()) {
+        if ($this->adapter->getSupportForSchemaIndexes()
+            && !($this->adapter->getSharedTables() && $this->isMigrating())) {
             $schemaIndexes = $this->getSchemaIndexes($collection->getId());
             $filteredId = $this->adapter->filter($id);