Bug: Nil Pointer Dereference in validateClustersOnUpdate

### 📜 Description

`validateClustersOnUpdate` dereferences `cluster.Status.ClusterHealth` without a nil guard. When a newly added cluster has not yet reported health status, `ClusterHealth` is `nil` and accessing `.ClusterHealthStatus` on it causes a nil pointer dereference panic.

**File:** `service/slice_config_webhook_validation.go`

### 👟 Reproduction steps

1. Register a new cluster that has not yet reported its health status (`ClusterHealth` is `nil`).
2. Create or update a `SliceConfig` that adds this cluster to `spec.clusters`.
3. The admission webhook calls `validateClustersOnUpdate`.
4. The controller panics of `slice_config_webhook_validation.go`.

### 👍 Expected behavior

It should validate gracefully - either skip the health check when `ClusterHealth` is `nil` , or return a clear validation error.

### 👎 Actual Behavior

It panics with a nil pointer dereference, crashing the webhook admission handler and blocking all `SliceConfig` updates.

### 🐚 Relevant log output

```shell
panic: runtime error: invalid memory address or nil pointer dereference
goroutine [running]:
github.com/kubeslice/kubeslice-controller/service.validateClustersOnUpdate(...)
service/slice_config_webhook_validation.go
```

### Version

_No response_

### 🖥️ What operating system are you seeing the problem on?

_No response_

### ✅ Proposed Solution

_No response_

### 👀 Have you spent some time to check if this issue has been raised before?

- [x] I checked and didn't find any similar issue

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct