fix: report direct ssh login failures

exec/lib/RemoteLoginUrlClient.php

@@ -96,7 +96,7 @@ final class RemoteLoginUrlClient
             throw new RuntimeException('Invalid process runner result');
         }
         if ($result->exitCode !== 0) {
-            throw new RuntimeException('MX helper failed');
+            throw new RuntimeException(self::failureMessage($settings, $result));
         }
         if (!preg_match('/https:\/\/\S+/', $result->stdout, $match)) {
             throw new RuntimeException('MX helper did not return a login URL');
@@ -108,6 +108,24 @@ final class RemoteLoginUrlClient
         }
     }
 
+    private static function failureMessage(Settings $settings, ProcessResult $result): string
+    {
+        $message = $settings->mxLoginMode() === 1 ? 'MX direct SSH command failed' : 'MX helper failed';
+        $message .= ' (exit ' . $result->exitCode . ')';
+        $detail = self::safeDiagnostic($result->stderr);
+        if ($detail !== '') {
+            $message .= ': ' . $detail;
+        }
+        return $message;
+    }
+
+    private static function safeDiagnostic(string $text): string
+    {
+        $text = preg_replace('/https?:\/\/\S+/i', '[redacted-url]', $text) ?? '';
+        $text = preg_replace('/\s+/', ' ', trim($text)) ?? '';
+        return substr($text, 0, 512);
+    }
+
     /**
      * @param list<string> $command
      */