fix: handle client-level Collection Exists exception in Mongo processException

The Mongo client throws Exception('Collection Exists') with code 0
rather than using MongoDB's native error code 48. This was only caught
by the pre-check, leaving the in-transaction path unhandled.

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

Author: abnegate

src/Database/Adapter/Mongo.php

@@ -3520,8 +3520,8 @@ protected function processException(\Throwable $e): \Throwable
             return new DuplicateException('Document already exists', $e->getCode(), $e);
         }
 
-        // Collection already exists
-        if ($e->getCode() === 48) {
+        // Collection already exists (code 48 from MongoDB, code 0 from client pre-check)
+        if ($e->getCode() === 48 || ($e->getCode() === 0 && stripos($e->getMessage(), 'Collection Exists') !== false)) {
             return new DuplicateException('Collection already exists', $e->getCode(), $e);
         }