fix: address PR #662 review comments (bus drain, context timeouts, onebot leak)

- Drain buffered messages in MessageBus.Close() so they aren't silently lost
- Replace all context.TODO() with context.WithTimeout(5s) across 7 call sites
- Fix OneBot pending channel leak: send nil sentinel in Stop() and handle
  nil response in sendAPIRequest() to unblock waiting goroutines

pkg/heartbeat/service.go

@@ -308,7 +308,9 @@ func (hs *HeartbeatService) sendResponse(response string) {
 		return
 	}
 
-	msgBus.PublishOutbound(context.TODO(), bus.OutboundMessage{
+	pubCtx, pubCancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer pubCancel()
+	msgBus.PublishOutbound(pubCtx, bus.OutboundMessage{
 		Channel: platform,
 		ChatID:  userID,
 		Content: response,