The QQCloseError (non-4008) reconnect path in _listen_loop was
missing the MAX_RECONNECT_ATTEMPTS upper-bound check that exists
in both the Exception handler (line 546) and the 4008 rate-limit
handler (line 486). Without this check, if _reconnect() fails
permanently for any non-4008 close code, backoff_idx grows
indefinitely and the bot retries forever at 60-second intervals
instead of giving up cleanly.
Fix: add the same guard after backoff_idx += 1 in the general
QQCloseError branch, consistent with the existing Exception path.