Fix frequent MQTT_Ping calls in handleKeepAlive

Related to #1915

Update `handleKeepAlive` in `source/core_mqtt.c` to correctly update `pContext->lastPacketTime` when data is received.

* Ensure `MQTT_Ping` is not called frequently due to `pContext->lastPacketTime` not updating.
* Retain the logic for sending ping packets based on both received and sent message intervals.
* Address the issue related to the underlying transport failing to send the data, ensuring reliable data transmission.