FleetClient calls tool-definition-controller:8788 /sessions/{id}/assign
as the primary sandbox create path. Falls back to direct k8s SandboxManager
if fleet controller is unreachable.
handleSandboxCreate:
1. fleet.Assign() → podSvcDns from pre-warmed fleet pod (fast, no wait)
2. fallback: sandbox.CreateOrResume() → dedicated pod (slow, 30s wait)
handleSandboxDelete: fleet.Release() + sandbox.Delete() both run (non-fatal).
updateSandboxSummary: extracted helper, updates session memory_entity with
pod_name / svc_dns / last_active_at.
NetworkPolicy: added port 8788 to poimen egress (fleet controller REST API).
Response adds fleet:true field when served by fleet.
