feat(): POC Active/Standby HA foundation for the controller

[Ady0333]

Description

The KubeSlice controller currently runs as a single replica, if that pod crashes, is evicted, or
its node fails, the entire multi-cluster control plane stalls (no SliceConfig reconciliation, no
gateway provisioning, no VPN key rotation) until the pod is rescheduled and its cache warms up cold.

This PR introduces the foundational layer for an Active/Standby topology, where multiple controller
replicas run concurrently: exactly one is Active and reconciles, while the others stay Standby
warm, observing state, and ready to take over within one lease period.

New package pkg/ha:

• LeaderElection :- coordination/v1 Lease-based election via client-go; releases the lease on context cancellation for fast failover; callback-driven transitions.
• StateSynchronizer :- periodically collects KubeSlice CRDs into a revisioned Snapshot and computes
  the add/remove/update SnapshotDiff vs. the prior revision, keeping a Standby warm.
• HealthChecker :- monitors the Active controller's lease renewal; flags it unhealthy only after a
  configurable threshold of consecutive failures (debounces transient API errors); panic-safe for
  leases missing optional fields.
• HAManager :- orchestrates the three components, owns ControllerState, drives Active↔Standby
  transitions, triggers an immediate resync on promotion, and exports Prometheus metrics.
• ActiveGuard :- a reconcile.Reconciler wrapper that drops requests on a Standby without mutating
  cluster state; controller-runtime's resync replays the work once the instance is promoted.

ResourceCollector / LeaseReader / ILeaderElection are narrow interfaces so the logic is fully
unit-testable without a live API server.

This is a self-contained, additive POC no existing controller wiring is modified.

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How Has This Been Tested?

34 unit tests run under the race detector and go vet, exercising real synchronizer/health/failover
logic via in-memory ResourceCollector and LeaseReader implementations (no stubs).

  go test -race ./pkg/ha/...
  ok    github.com/kubeslice/kubeslice-controller/pkg/ha        1.201s   (34/34 pass)

  go vet ./pkg/ha/...  

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Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.

Does this PR introduce a breaking change for other components like worker-operator?

No. The change is purely additive a new pkg/ha package and documentation, with no modifications to existing controllers, CRDs, or APIs.