[FEATURE] objectstore-plugin requires an explicit ATE_STORAGE_BACKEND #3668

The base manifests select GCS as the storage backend, and the plugin treats any value other than s3, including unset, as GCS. A default install on a non-GCP cluster silently uses GCS.

Tracks "Require an explicit object-store backend" in the Provider-Neutral Core workstream of the Extensible Install Roadmap.

Current state (0b635b80)

• ATE_STORAGE_BACKEND: "gcs" in manifests/ate-install/atelet.yaml and manifests/ate-install/ate-api-server.yaml.
• pkg/objectstorage/env.go: any value other than s3 selects GCS, with no warning when unset.
• Open PRs keep the GCS default: objectstore-plugin: storage settings come from ate-storage-config #3197 (ate-storage-config) and ate-setup: install a provider's object store plugin with --component and storage.backend=external #3651 (storage.backend, default gcs).

Required changes (two steps)

1. Warning release: log a warning at plugin startup when ATE_STORAGE_BACKEND is unset or unknown. No behaviour change.
2. After the GKE parity release (GKE parity release: install through the GKE Component ai-on-gke/substrate-gke#151):
   • fail on an unset or unknown value;
   • remove gcs from the base manifests and from any ate-setup default (including storage.backend if ate-setup: install a provider's object store plugin with --component and storage.backend=external #3651 lands);
   • the kind overlay keeps s3, and the GKE Component sets gcs (Complete the GKE kustomize Component ai-on-gke/substrate-gke#152).

Dependencies: step 2 waits for ai-on-gke/substrate-gke#151.

Breaking change: step 2. Release note: installs must name a storage backend.

Enforcement

• Unit tests for unset, unknown, s3 and gcs.
• Render test: the base sets no backend; the kind render sets s3.

Parent Issue: #3161