bug: StatefulSet spec.selector is immutable and cannot be updated in CopyStatefulSetFields

### Description

In `workspaces/controller/internal/helper/helper.go`, the `CopyStatefulSetFields` function attempts to copy and update `spec.selector` on existing StatefulSets:

```go
// copy `spec.selector`
//
// TODO: confirm if StatefulSets support updates to the selector
// if not, we might need to recreate the StatefulSet
//
if !equality.Semantic.DeepEqual(target.Spec.Selector, desired.Spec.Selector) {
target.Spec.Selector = desired.Spec.Selector
requireUpdate = true
}
```

However, Kubernetes StatefulSets do not support updates to `spec.selector` because `spec.selector` is immutable after creation.

### Kubernetes Reference

According to upstream Kubernetes [`staging/src/k8s.io/api/apps/v1/types.go#L214`](https://github.com/kubernetes/kubernetes/blob/73b08be6346d68bf550c95789fdde1a305dd0820/staging/src/k8s.io/api/apps/v1/types.go#L214) and API validation in `pkg/apis/apps/validation/validation.go`:
- StatefulSet `spec.selector` is an immutable field (`apivalidation.ValidateImmutableField(...)`).
- Any attempt to modify `spec.selector` in an update request is rejected by the Kubernetes API server with a 422 Invalid error: `spec.selector: Invalid value: ...: field is immutable`.

### Impact

When `CopyStatefulSetFields` detects a difference in `target.Spec.Selector` vs `desired.Spec.Selector`, it sets `target.Spec.Selector = desired.Spec.Selector` and returns `requireUpdate = true`.

When `workspace_controller.go` attempts to apply this update via `r.Update(ctx, foundStatefulSet)`:
```go
if helper.CopyStatefulSetFields(statefulSet, foundStatefulSet) {
if err := r.Update(ctx, foundStatefulSet); err != nil {
log.Error(err, "unable to update StatefulSet")
return ctrl.Result{}, err
}
}
```
The API server rejects the update, causing the controller to enter a failing reconciliation loop.

### Suggested Fix

1. Remove updating `target.Spec.Selector` in `CopyStatefulSetFields`, as in-place updates to `spec.selector` are not allowed by the Kubernetes API server.
2. If selector updates need to be supported, the controller should recreate (delete and create) the StatefulSet.
3. If selector updates are not supported, consider detecting selector mismatches and logging an appropriate error or handling recreation rather than attempting an invalid `r.Update`.