Enable verbose Kubernetes health checks

[mterwill]

Summary

• request verbose Kubernetes health checks from the cluster monitor
• unwrap Kubernetes health endpoint status errors so verbose output is readable
• include compact livez/readyz failure details in cluster status errors instead of dumping every successful check
• record verbose health-check calls in the fake K8s client and cover the behavior in tests

Tests

• go test ./internal/controllers/core/cluster
• go test ./internal/k8s