Don't set the entire Networking field in AgentClusterInstallSpec

This was attempting to set the `UserManagedNetworking` field to nil
when it had previously been defaulted to some value by the
assisted-service controllers.

This error can be seen in logs like:

```
2025-04-03T16:17:55Z	ERROR	Reconciler error	{"controller": "clusterdeployment", "controllerGroup": "hive.openshift.io", "controllerKind": "ClusterDeployment", "ClusterDeployment": {"name":"test-multinode","namespace":"test-capi"}, "namespace": "test-capi", "name": "test-multinode", "reconcileID": "316eee13-3248-4919-9569-f40026a9a95f", "error": "admission webhook \"agentclusterinstallvalidators.admission.agentinstall.openshift.io\" denied the request: Attempted to change AgentClusterInstall.Spec which is immutable after install started, except for ClusterMetadata,IgnitionEndpoint fields. Unsupported change: \n\tNetworking.UserManagedNetworking: (0xc000988c4c => <nil>)"}
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.19.6/pkg/internal/controller/controller.go:316
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.19.6/pkg/internal/controller/controller.go:263
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func2.2
	/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.19.6/pkg/internal/controller/controller.go:224
```

Author: carbonin

controlplane/internal/controller/clusterdeployment_controller.go

@@ -156,10 +156,8 @@ func (r *ClusterDeploymentReconciler) ensureAgentClusterInstall(
 		aci.Spec.Proxy = oacp.Spec.Config.Proxy
 		aci.Spec.SSHPublicKey = oacp.Spec.Config.SSHAuthorizedKey
 		aci.Spec.ImageSetRef = &hivev1.ClusterImageSetReference{Name: clusterDeployment.Name}
-		aci.Spec.Networking = hiveext.Networking{
-			ClusterNetwork: clusterNetwork,
-			ServiceNetwork: serviceNetwork,
-		}
+		aci.Spec.Networking.ClusterNetwork = clusterNetwork
+		aci.Spec.Networking.ServiceNetwork = serviceNetwork
 		aci.Spec.ManifestsConfigMapRefs = additionalManifests
 
 		if len(oacp.Spec.Config.APIVIPs) > 0 && len(oacp.Spec.Config.IngressVIPs) > 0 {