feat(kubernetes): support driver config passthrough

[elezar]

Summary

Adds the driver_config path end to end: CLI raw JSON, gateway driver-key selection, and Kubernetes driver-local typed config consumption. The Kubernetes driver now supports pod scheduling fields, agent container resources, and pod.runtime_class_name mapped to PodSpec runtimeClassName.

The typed SandboxTemplate runtime_class_name remains highest precedence. Kubernetes driver_config pod.runtime_class_name overrides the gateway default_runtime_class_name when the typed template field is absent.

Related Issue

Discussion POC for #1589.

Related to #1492.

Does not close either reference.

Changes

• Adds a pre-POC Docker test cleanup that switches sandbox template fixtures to ..Default::default().
• Adds experimental --driver-config-json CLI plumbing and forwards the selected driver block through the gateway to the compute driver.
• Adds Kubernetes driver-local config handling for pod scheduling fields, agent container resources, and pod.runtime_class_name.
• Maps pod.runtime_class_name to PodSpec runtimeClassName with precedence over default_runtime_class_name.
• Documents the experimental CLI surface and Kubernetes config shape in the driver README and published docs.
• Notes in the Kubernetes driver that the typed config schema should remain driver-local and may later move to driver-local proto definitions.

Testing

• cargo test -p openshell-driver-kubernetes
• mise run pre-commit
• Unit tests added/updated
• E2E tests added/updated (not run; covered by unit tests and pre-commit for this PR)

Checklist

• Follows Conventional Commits format
• Commits are signed off (DCO)
• Documentation updated (driver README and published docs)
• Architecture docs updated (not applicable; published/reference docs updated instead)

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[github-actions]

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[drew]

This looks really good to me. I say we ship it and then add podman/docker volume configs, and gpu configs.

Once those are in maybe we can work on config validation and more concretely defining schemas that can be validated by drivers.