Update src/Core_Command.php

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: swissspidy

src/Core_Command.php

@@ -1803,6 +1803,7 @@ private function cleanup_extra_files( $version_from, $version_to, $locale, $inse
 		$has_checksums = is_array( $old_checksums ) && is_array( $new_checksums );
 
 		if ( ! $has_checksums ) {
+			WP_CLI::warning( 'Could not retrieve WordPress core checksums; skipping checksum-based cleanup. Files listed in $_old_files were still cleaned up.' );
 			return;
 		}