[BUG] Reject negative storageRequestSize in k3k

Environmental Info:

• Host Cluster Version: 1.35.2+rke2r1
• K3KCLI Version: 1.1.0-rc2
• K3K Helm Chart Version: 1.1.0-rc2
• K3K Cluster Configuration: 1 server, shared mode

Note: This was observed when validating https://github.com/rancher/k3k/issues/560

Describe the bug: Currently, k3kcli and the Cluster CR allow creating clusters with negative storage sizes. Cluster creation is attempted, but it ultimately fails with the following error: StatefulSet.apps "k3k-<cluster>-server" is invalid: spec.volumeClaimTemplates[0].spec.resources[storage]: Invalid value: "-1G": must be greater than zero It would be better if the CLI/API rejected negative values upfront.

Steps To Reproduce:

1. Create a cluster with negative storage size: k3kcli cluster create -n k3k-system --mode shared --storage-class-name longhorn --storage-request-size -1G --kubeconfig-server <server> testcluster

Expected behavior:

• CLI/API validates storageRequestSize before creating the cluster.
• Negative values are rejected immediately.

Actual behavior:

• CLI/API does not reject negative values upfront.
• Cluster creation fails only after attempting resource creation.

$ k3kcli cluster create -n k3k-system --mode shared --storage-class-name longhorn --storage-request-size -1G --kubeconfig-server <server> k3ksharedlonghorn8 --debug
INFO[0000] Creating cluster 'k3ksharedlonghorn8' in namespace 'k3k-system' 
INFO[0001] Cluster details:
 Mode: shared
 Servers: 1
 Version: v1.35.2 (Host: v1.35.2)
 Persistence:
  Type: dynamic
  StorageClass: longhorn
  Size: -1G 
INFO[0001] Waiting for cluster to be available..     
Error: failed to wait for cluster to become ready (status: Failed): cluster creation failed: Failed
FATA[0001] failed to wait for cluster to become ready (status: Failed): cluster creation failed: Failed 

$ k3kcli cluster list | grep k3ksharedlonghorn8
k3k-system  k3ksharedlonghorn8  shared  Failed     <none>

$ kubectl describe clusters.k3k.io -n k3k-system k3ksharedlonghorn8 | tail -5
  k3k-k3ksharedlonghorn8-service.k3k-system
Events:
 Type   Reason       Age        From          Message
 ----   ------       ----        ----          -------
 Warning ProvisioningFailed 0s (x11 over 24s) k3k-cluster-controller StatefulSet.apps "k3k-k3ksharedlonghorn8-server" is invalid: spec.volumeClaimTemplates[0].spec.resources[storage]: Invalid value: "-1G": must be greater than zero

$ kubectl get pvc -n k3k-system | grep k3ksharedlonghorn8
$

cc @bigkevmcd