fix: set TenantID on root volume created by RunInstances (mulga-758)

GenerateVolumes did not set TenantID on the VolumeMetadata for root
volumes cloned from AMIs. After the pre-Phase4 tenant fallback was
removed (ae316db), describe-volumes filters volumes by exact tenantID
match — root volumes with empty TenantID became invisible to the
owning account, breaking E2E Phase 5b volume lifecycle tests.

spinifex/handlers/ec2/instance/service_impl.go

@@ -258,6 +258,7 @@ func (s *InstanceServiceImpl) GenerateVolumes(input *ec2.RunInstancesInput, inst
 			IOPS:                p.iops,
 			SnapshotID:          p.snapshotId,
 			DeleteOnTermination: p.deleteOnTermination,
+			TenantID:            instance.AccountID,
 		},
 	}