feat: Add structured Kubernetes Events for sandbox lifecycle transitions

What would you like to be added:
Emit Kubernetes Events via client-go/tools/record.EventRecorder at key sandbox lifecycle
points in the Workload Manager. Currently, sandbox creation, readiness, rollback, and garbage
collection are tracked only through internal Go channels and klog statements, no Events are
emitted on the parent AgentRuntime or CodeInterpreter resources.

Why is this needed:

Emitting Events is standard Kubernetes controller practice, every core controller does it.
Without Events, operators using kubectl describe agentruntime/<name> see no history of what
happened to their sandboxes. Debugging a slow or failed sandbox startup today requires tailing
workload-manager logs directly rather than using kubectl get events.

This also becomes critical for the upcoming multi-AgentCube orchestration feature, where
a single task spans multiple coordinated sandboxes. Operators and platform teams will need a
reliable, per-resource audit trail to monitor lifecycle transitions at scale.