feat(k8s): support nodeSelector and tolerations from platform_config

[Arnonrgo]

Summary

• No way to schedule sandbox pods on specific node pools or tolerate taints
• Wire up node_selector and tolerations from the existing platform_config passthrough
• Callers can now target GPU node pools or tolerate NoSchedule taints

Changes

• Extract node_selector and tolerations from platform_config Struct in sandbox_template_to_k8s
• Apply them to the pod spec's nodeSelector and tolerations fields

Testing

• Unit test: nodeSelector extracted from platform_config and applied to pod spec
• Unit test: tolerations extracted from platform_config and applied to pod spec
• cargo check -p openshell-driver-kubernetes passes

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[Arnonrgo]

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[Arnonrgo]

recheck

[drew]

/ok to test 2769178

[drew]

/ok to test 8f4253b