fix: pass cluster K8s version to Helm SDK chart rendering

[mchmarny]

Summary

• Extract K8s server version from snapshot and pass to Helm SDK's install.KubeVersion during chart rendering
• Fixes chart requires kubeVersion: >= X which is incompatible with Kubernetes v1.20.0 errors in validation

Root cause

PR #186 replaced the helm CLI subprocess with the Helm Go SDK for chart rendering. In client-only mode, the SDK copies chartutil.DefaultCapabilities which hardcodes k8sVersionMajor=1, k8sVersionMinor=20 (Kubernetes v1.20.0). The install.KubeVersion override was never set, so charts with kubeVersion constraints (cert-manager, network-operator, skyhook-operator, etc.) fail rendering.

Fix

• Add kubeVersion parameter to resolveExpectedResources() and renderHelmTemplate()
• New kubeVersionFromSnapshot() helper extracts the version from the K8s server subtype
• Set install.KubeVersion via chartutil.ParseKubeVersion() before RunWithContext()
• When no snapshot is available (tests), empty string preserves existing default behavior

Test plan

• go test -race ./pkg/validator/... — all tests pass
• make test — full suite passes
• make lint — 0 issues
• CI GPU Inference Test should now render all charts successfully