🐛 [scanner] fix: add nil guards for k8s client in API handlers

[kubestellar-hive]

Fixes #18246

Adds nil guards for the k8s MultiClusterClient in kagenti provider proxy handlers that were panicking with nil pointer dereferences:

• handleGetClusterList - guards against nil before calling DeduplicatedClusters
• handleGetPodList - guards against nil before calling GetPods
• handleGetEvents - guards against nil before calling GetEvents

All three handlers now return 503 Service Unavailable when the k8s client is not initialized, consistent with the pattern used in other handlers like admission_webhooks.go, lima.go, and crds.go.

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