Adds guest-side GCS changes for V2 shim support

[shreyanshjain7174]

Summary

Guest-side GCS changes to support V2 shim path layouts and multi-pod UVMs. Replaces hard-coded path derivation with OCIBundlePath as the source of truth, and unifies the separate V1/virtual-pod/V2 pod tracking into a single code path.

This issue tracks the guest work PRs for the V2 shim.

Pending Review

• guest: use OCIBundlePath as sandbox root source of truth #2653
• guest: unify pod model for V1, virtual pod, and V2 shim support #2662