feat(server): add sandbox k8s runtime

[Generalwin]

Summary

What is changing and why?
This PR implements Kubernetes runtime support for OpenSandbox, enabling sandboxes to run on K8s clusters.
Key Components:

• KubernetesSandboxService: K8s implementation of sandbox service interface
• BatchSandboxProvider: Workload provider using custom BatchSandbox CRD
• WorkloadProvider: Abstract interface for extensibility (Pod/Job/StatefulSet support in future)
• ProviderFactory: Factory pattern with dynamic provider registration
• K8sClient: Client wrapper supporting kubeconfig and in-cluster auth
• BatchSandboxTemplateManager: YAML template manager with deep merge

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

[CLAassistant]

All committers have signed the CLA.

[Pangjiping]

missing license header, run scripts/add-license.sh under project root.

https://github.com/alibaba/OpenSandbox/actions/runs/20503579182/job/58915498419?pr=47

[Pangjiping]

error in python format, run uv run ruff check --fix to fix it

[Pangjiping]

LGTM