fix: Wait for the cache to show a StatefulSet shrink before the next pass

[as51340]

Problem

replicaCounts applies max(declared, cached spec.replicas). After shedRetiredPods lowers a StatefulSet, the next pass can still read the pre-shrink copy from the informer cache. The status write that ends the shedding pass requeues the cluster on the MemgraphCluster watch stream, which isn't ordered with the StatefulSet stream. That pass then:

1. applies the old count again, so the StatefulSet controller may recreate the shed pod, and
2. finds the member already unregistered or removed, and shrinks the StatefulSet a second time.

The end state is correct, so this is churn rather than data loss. But a retired coordinator or data instance pod can briefly come back on its retained volume.

Fix

This is the Kubernetes expectations pattern, shaped like ECK's ExpectedStatefulSetUpdates and narrowed to the one write whose stale read undoes it:

• New internal/expectations: StatefulSets.Expect(name, uid, generation) and Satisfied(name, cached). An expectation is satisfied once the cached copy is gone, is a different object, or has reached the generation. It's forgotten once satisfied.
• shedRetiredPods records the UID and generation its apply returned. To get them, apply now returns the written object, and appliedGenerations became appliedObjects.
• replicaCounts checks each role's cached StatefulSet. If either is behind, the pass ends before applying anything and returns RequeueAfter: requeueWhilePending. The Owns StatefulSet event normally requeues it first.
• The expectation is held in memory only. A restarted operator lists into a fresh cache that already holds the shrink, so only the process that wrote it can be ahead of its own cache.

Every other cached read stays as it is. The apply restates desired state, pod deletes carry a UID precondition, rollout's Stale check covers status, and membership is read live over Bolt. CLAUDE.md gets a convention bullet with this reasoning.

Tests

• Unit tests in internal/expectations cover older and newer generations, a missing StatefulSet, a replaced UID, forgetting a satisfied expectation, and a later write replacing an earlier one.
• A new envtest serves the pre-shrink StatefulSet through a laggingCache client, then checks that the StatefulSet's generation doesn't change and that the pass converges once the cache catches up. With the gate disabled the test fails: the generation goes 2 → 4, grown back and then shrunk again.
• make test and make lint pass.