fix(bufstream): Clean up etcd PVCs during W&B deletion

[danielpanzella]

Deleting a W&B CR with retentionPolicy.onDelete: purge can leave Bufstream's etcd PVCs behind because the old claim template lacks the W&B labels used by the purge selector. Separately, the normal read/reconcile path attempts to purge etcd data whenever the broker Application is missing, even during a partial deployment.

This change makes the existing W&B deletion finalizer the only Bufstream PVC cleanup path:

• Remove PVC deletion from ReadState; a missing broker Application reports a missing resource and preserves etcd data.
• Add W&B purge labels to new etcd claim templates, and recognize legacy claims using the exact workload labels and generated claim name while rejecting conflicting ownership labels.
• Preserve detach and per-component retention overrides. A failed PVC deletion keeps the W&B cleanup finalizer so reconciliation can retry.

Legacy claim matching retains the workload-label checks: Kubernetes copies StatefulSet selector labels onto generated PVCs even when the claim template has no labels (upstream controller). This matches the saved WESTest claims; unlabeled, unrelated claims are not deleted by name alone.

Validation:

• Added a regression test that reproduced deletion of new and legacy claims during normal reconciliation before the fix and now passes.
• Added tests through the W&B Reconcile deletion path for purge with Applications present/absent, detach, retention overrides, finalizer ordering, and retry after a deletion error.
• Bufstream and reconciler package suites pass, including the new regressions. go vet ./... passes.
• make test ran the full suite; its only failure was the unrelated TestTelemetryChartFullModeRendersCoreStack, whose Helm output is missing name: victoria-log-agent.
• make lint could not complete: installed golangci-lint 2.13.2 cannot decode the Go 1.27.2 export data format (version 5 is greater than maximum supported version 4).

A published operator build and fresh EKS end-to-end validation are still pending. The companion WESTest PR #56 adds PVC/PV draining and run-scoped EBS cleanup.