[BUG] CacheEngine does not populate Status.Selector, breaking HPA and worker pod discovery

[adityaupasani2]

What's wrong

CacheEngine.CheckAndUpdateRuntimeStatus() in pkg/ddc/cache/engine/status.go has an unresolved TODO:

// TODO(cache runtime): set the CacheRuntime Status left fields: Selector
runtimeToUpdate.Status.ValueFile = common.GetCacheRuntimeConfigConfigMapName(e.name)

And in pkg/ddc/cache/engine/master.go, the line that would set it is commented out:

// TODO(cache runtime): figure out how to use this selector
// runtimeToUpdate.Status.Selector = e.getWorkerSelectors()

Status.Selector is a label selector string used for worker pod discovery and HPA (Horizontal Pod Autoscaler) integration. Every other runtime populates it — AlluxioEngine, JindoCacheEngine, JuiceFSEngine, VineyardEngine, EFCEngine all call e.getWorkerSelectors() during master setup. CacheRuntime never sets it, so status.selector is always empty.

Impact

Any tooling or controller that reads CacheRuntime.Status.Selector to discover worker pods — including HPA and the Fluid scheduler plugin — will see an empty string and fail to match any pods.

Proposed fix

1. Add getWorkerSelectors() to CacheEngine using the labels set by getCommonLabelsFromComponent() — specifically common.LabelCacheRuntimeName and common.LabelCacheRuntimeComponentName with component type worker
2. Call it during master setup in SetupMaster() to set runtimeToUpdate.Status.Selector
3. Remove the two TODO comments

I'd like to work on this.