post https://example.com/ServerAgent/ValidateAgent
This acutally does same as AgentBackChannelManager::GetAgent().
This is for the case when cagent found health/ping works yet the agent is
not existing in backend anymore (such as deleted from admin portal),
this call will ensure that cagent is really valid, like the 'agent ping'.
Keep this under 'Update' region, for no better places..