Summary

• Backport DDC graceful rollout hardening to the 25.8 branch
• Keep DDC compute group StatefulSets on OnDelete and drive BE replacement through the operator-controlled graceful rollout flow
• Preserve default podinfo mounts in DDC FE/BE/MS containers and remove the old DownwardAPI compensation path to avoid pod template drift
• Add tests for podinfo mounts and rollout state handling

Tests

• go test ./pkg/controller/sub_controller/disaggregated_cluster/computegroups ./pkg/controller/sub_controller/disaggregated_cluster/disaggregated_fe ./pkg/controller/sub_controller/disaggregated_cluster/metaservice ./pkg/common/utils/resource