fix(backup): add file existence check for wp-config backup

Add missing file existence check in backup_wp_content_dir() for
consistency with other backup functions. This ensures the archive
was actually created even when 7z returns exit code 0 or 1.

Addresses GitHub Copilot review feedback on PR #473.

Author: mrrobot47

src/helper/Site_Backup_Restore.php

@@ -480,7 +480,7 @@ private function backup_wp_content_dir( $backup_dir ) {
 		$result         = EE::launch( $backup_command );
 
 		// 7z exit codes: 0=success, 1=warning (non-fatal), 2+=fatal error
-		if ( $result->return_code >= 2 ) {
+		if ( $result->return_code >= 2 || ! $this->fs->exists( $backup_file ) ) {
 			$this->capture_error(
 				'Failed to create WordPress content backup archive',
 				self::ERROR_TYPE_FILESYSTEM,