feat(heal): optionally protect the redis master from autoscaler eviction

[binhnguyenduc]

What

Adds an opt-in redis.preventMasterEviction flag. When enabled, the operator annotates the current master pod with cluster-autoscaler.kubernetes.io/safe-to-evict: "false" and marks slaves "true", so the Kubernetes cluster-autoscaler won't drain the node running the master (which would force an avoidable failover). The annotation follows the master as the role moves — applied in both the checker and healer relabel paths.

Why

Cluster-autoscaler scale-down can drain the node hosting the redis master, triggering an unnecessary failover and a brief write outage. Pinning only the master (slaves stay evictable) lets the autoscaler still consolidate the rest of the pool.

Design

• Opt-in (default ): always-on would silently pin a node from scale-down for every user. Off = annotations are never touched.
• New Pod.UpdatePodAnnotations service method uses a JSON merge patch (creates the annotations map if absent), mirroring the existing targeted UpdatePodLabels patch rather than a full-object update.
• The annotation helper reads desired state off the already-fetched pod and skips the patch when already correct, so it's safe to run every reconcile without extra API writes and self-corrects if the flag is toggled on after the master exists.

Note on the mock

Adding the interface method requires regenerating mocks/service/k8s. The committed mocks are mockery v2.20.0, which panics against the current Go 1.25 toolchain, so the new mock method is hand-written in the exact v2.20.0 format (a future correct regen is diff-free). Mocks are not part of the verify-generated CI check.

Changes

• api/.../types.go: RedisSettings.PreventMasterEviction (+ regenerated CRD).
• service/k8s/pod.go: UpdatePodAnnotations + mock.
• check.go / heal.go: shared applyMasterEvictionAnnotation wired into both relabel paths.
• Tests for the annotation logic; README section.

Upstream references

• Adding Support for annotation for cluster autoscaler to skip master eviction spotahome/redis-operator#689 (annotation for cluster autoscaler to skip master eviction)

Test

go build ./..., go test ./..., gofmt clean; CRD regenerated via pinned codegen image.