Upgrade agent-sandbox to v0.5.2 or a later stable release

What would you like to be added:

Upgrade AgentCube from sigs.k8s.io/agent-sandbox v0.4.6 to v0.5.2 or a later stable release.

PR #387 intentionally stopped at v0.4.6 because agent-sandbox v0.5 introduces a broader v1alpha1 to v1beta1 API and storage migration. This follow-up should investigate the required compatibility changes, update the integration, and add appropriate unit and E2E coverage.

The upgrade should preserve existing direct Sandbox and CodeInterpreter warm-pool behavior, including SandboxClaim adoption, session deletion, garbage collection, and pool refill.

Existing v0.4.6 installations and active SandboxClaims should also have a documented and tested upgrade path.

References:

• #386
• #387
• https://github.com/kubernetes-sigs/agent-sandbox/blob/main/docs/api-migration-guide.md
• https://github.com/kubernetes-sigs/agent-sandbox/pull/1124

Why is this needed:

AgentCube currently depends on v0.4.6 and still uses the upstream v1alpha1 APIs. Moving to the latest stable release keeps AgentCube aligned with the supported agent-sandbox API and includes important correctness and security fixes added after v0.4.6.

Upstream warns that v0.5.0 and v0.5.1 can disrupt active warm-started claims during an upgrade. The target version must therefore be v0.5.2 or a later stable release containing the fix from kubernetes-sigs/agent-sandbox#1124.