When an agent misses a checkin, why do we call it unhealthy vs. offline?

Would be great to revisit [this](https://github.com/elastic/elastic-agent/blob/5ca0ae1c94ac35e5d458b32ec0ad715e9f08a83f/internal/pkg/agent/application/gateway/fleet/fleet_gateway.go/#L294-L312) code.

We might be treating `offline` agents as `unhealthy` for no real reason...