Enforce singleton GPUCluster name and select ClusterPolicy by creation time

[karthikvetrivel]

With multiple CRs of a kind, controllers resolved "the active one" inconsistently: the owning controllers kept a first-reconciled-wins claim in memory (unstable across operator restarts, invisible to other controllers), while resolveActiveConfig picked the first list item. This PR makes the singleton deterministic for both kinds:

• GPUCluster: a CEL rule on the CRD pins metadata.name to gpu-cluster (the name the Helm chart creates), enforcing the singleton at admission. That makes the controller's runtime singleton handling dead code, so it is removed: the in-memory first-wins claim and the ignored status value are gone, and the sample is renamed to gpu-cluster. GPUCluster has not shipped, so there is no upgrade path to consider.
• ClusterPolicy: nothing enforces a singleton at admission, so the instance with the oldest creationTimestamp (name as tie-breaker) is treated as the singleton, via a new getSingletonClusterPolicy helper used by the ClusterPolicy controller, resolveActiveConfig, and the GPUCluster prerequisite check (which now inspects only the singleton ClusterPolicy). This selection is a heuristic: ideally the operator would hold a reference to the singleton ClusterPolicy globally throughout its lifetime, and until it does, picking the oldest is the next-best approach.

Deleting the singleton ClusterPolicy promotes the next-oldest lazily (on its next reconcile), same as today.

[tariq1890]

@karthikvetrivel Can we amend the PR title and commit as well?

Note, the reason I am being particular here is that we don't really elect a ClusterPolicy in the way that distributed systems elect a "leader" or "primary actor". Secondly, let'd mention in the comments that the selection of the oldest clusterPolicy as the singleton is a heuristic. Ideally, we should have a way of holding a reference to the singleton ClusterPolicy globally throughout the operator's lifetime. Since we don't have this right now, selection of the ClusterPolicy by the oldestTimestamp is the next best approach.