Reject invalid cluster node health values instead of defaulting to online

## Problem

`ClusterExtendedNodeInfo::health` defaults to `ONLINE`. When `health` is not a string or contains
an unknown value, `ParseClusterNode` logs an error but still returns the node.

For example, `"health": "faill"` is accepted as `ONLINE`. The replica may consequently appear in
`CLUSTER SLOTS` as a healthy read target.

## References

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/src/server/cluster/cluster_defs.h#L108-L116

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/src/server/cluster/cluster_config.cc#L260-L282

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/docs/cluster-node-health.md#L34-L48

Start with ParseClusterNode in src/server/cluster/cluster_config.cc and compare its handling with ClusterExtendedNodeInfo::health in src/server/cluster/cluster_defs.h. Check the documented health values in docs/cluster-node-health.md. Done means non-string or unknown health values are rejected instead of producing an online node, while valid values retain their documented behavior.

Written by the indexing model from the issue text.