[Server]: Report if container runtime is running in health endpoint

[alex-mcgovern]

Related to this bug for ToolHive Studio: https://github.com/StacklokLabs/toolhive-react/issues/81

If we try to start the server and a supported container runtime is not started, thv panics:

./bin/darwin-arm64/thv serve
8:59AM ERR failed to create lifecycle manager: no supported container runtime available: container runtime not found
panic: failed to create lifecycle manager: %v

The request is to allow the server to start without the container runtime (defer this check to request time?), and instead report the status of the container runtime via the /health endpoint.