feat(lifecycle): expose confirmed pool allocation summary

[cwj2001]

Problem

The lifecycle API accepts extensions.poolRef, but GET /sandboxes/{id} currently does not expose whether the Kubernetes Pool controller actually confirmed a Pool allocation. The request value is intent rather than proof, especially for poolRef: "*" auto-assignment.

This prevents HTTP-only control planes and platform operators from distinguishing a confirmed Pool allocation from an unconfirmed or pending state without Kubernetes RBAC or parsing controller-internal annotations.

Proposal

Add an optional, additive allocation summary to the existing lifecycle Sandbox response:

{
  "allocation": {
    "mode": "pool",
    "poolRef": "example-pool",
    "state": "allocated"
  }
}

The Server should emit it only when its Kubernetes runtime can confirm the current Pool allocation from its internal workload/controller state. Omit it for direct sandboxes, unsupported providers, pending or inconsistent allocation, release/deletion, or otherwise unconfirmed state. Absence must not be interpreted as cold-create, pending, or unsupported.

The summary intentionally excludes Pod names, namespace, node/IP/UID, raw annotations/finalizers, allocation history, and requested Pool reference. It is not a new endpoint, does not change CreateSandbox response, CRDs, or controller behavior.

Reusable users

• HTTP-only Gateways/control planes that deliberately have no Kubernetes RBAC.
• poolRef: "*" callers that need the effective Pool once allocation is confirmed.
• Self-hosted platform operators diagnosing capacity/allocation failures.
• Performance/regression tooling that must distinguish confirmed warm-pool allocation from non-evidence.

Compatibility

Optional response-only field; existing clients can ignore it. The implementation should update the lifecycle OpenAPI source, Server mapper/tests, and affected generated SDKs.

I plan to submit a minimal implementation PR if maintainers agree this belongs in the lifecycle resource contract.