feat: support manual cleanup sandboxes

[hittyt]

Summary

• Add manual cleanup sandbox lifecycle support by treating omitted or null timeout as non-expiring sandbox creation.
• Update Docker lifecycle handling, timeout limit validation, SDKs, tests, and docs to support nullable expiresAt and explicit cleanup semantics.
• Document the controlled rollout path: upgrade SDKs first, then upgrade the server, then start creating manual-cleanup sandboxes.

Testing

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

Breaking Changes

• None
• Yes (describe impact and migration path)

Impact:

• Lifecycle API responses may now return expiresAt = null for manual-cleanup sandboxes.
• Older SDKs that assume expiresAt is always a timestamp may fail when they create, get, or list manual-cleanup sandboxes.

Migration path:

• Upgrade SDKs/clients first.
• Upgrade the server second.
• Only then start creating sandboxes with timeout omitted or null.
• Do not introduce manual-cleanup sandboxes into shared environments while older lifecycle API readers are still active.

Checklist

• Linked Issue or clearly described motivation Proposal: Support non-expiring sandboxes / manual cleanup mode across Docker and Kubernetes #442
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

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[Pangjiping]

LGTM. I'll be working with @Generalwin to get the kubernetes_service work done.