fix: reject ddc compute group storage template changes

[Al-assad]

Motivation

Changing BE file cache paths or persistent volume settings after a DDC compute group has been created changes the generated StatefulSet volume claim templates. Kubernetes does not allow updating StatefulSet volumeClaimTemplates in place, so the reconcile currently fails later with a low-level immutable field error. This makes the failure hard to understand and can look like a generic StatefulSet apply problem.

This change detects the storage template change before applying the StatefulSet and returns a clear operator event telling users to recreate the compute group for this kind of storage layout change.

Summary

• reject DDC compute group storage template changes before applying StatefulSet
• return a clear CGStorageTemplateImmutable event for file_cache_path / persistent volume changes
• add unit coverage for immutable storage template detection and default volumeMode normalization

Tests

• go test ./pkg/controller/sub_controller/disaggregated_cluster/computegroups

Notes

• go test ./pkg/controller/sub_controller/... currently requires local envtest binaries; this environment is missing bin/k8s/1.26.1-darwin-arm64/etcd.