The checkHealth method handles non-ok responses (e.g. 503) but only
network-level errors had test coverage. Add nonOkHealth helper and
test to verify connected → disconnected transition on HTTP 503.
checkHealth previously ignored non-ok responses (e.g. 503), silently
doing nothing when the server reported unhealthy. Add else branch to
transition connected → disconnected on non-ok status. Add tests for
connected → disconnected on fetch failure and offline → reconnected
on health check success.
SnapOtter