🐛 Fix MachinePool nodeRef UID mismatch after K8s upgrade

[jayesh-srivastava]

What this PR does / why we need it:
When a K8s upgrade is performed on a Managed cluster, new nodes will come up with new UIDs. However, the MachinePool controller has an early return condition that only validates the count of NodeRefs but doesn't check if the UIDs are still valid. This leads to MachinePools retaining stale NodeRef UIDs after upgrades, causing UID mismatches that persist until manual intervention.
This PR adds UID validation logic before the early return condition.

• A new name-to-node map with the name nodeNameMap is created.
• We iterate over the mp.Status.NodeRefs and using the above map, get each each Node.
• If the Node doesn't exists or the fetched Node's UID is not matching to the UID in NodeRef, we break and continue with further reconciliation. This will set the correct nodeRef in the Machine Pool.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #12388

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[k8s-ci-robot]

@jayesh-srivastava: The label(s) area/pool cannot be applied, because the repository doesn't have them.

In response to this:

/area machine pool

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[jayesh-srivastava]

/area machinepool

[mboersma]

It seems reasonable to add this additional verification in general, but I wonder if this problem has been seen in providers other than CAPZ.

[mboersma]

Suggested change

	nodeNameMap := make(map[string]*corev1.Node)
	nodeNameMap := make(map[string]*corev1.Node, len(s.nodeRefMap))

[sbueringer]

It seems reasonable to add this additional verification in general, but I wonder if this problem has been seen in providers other than CAPZ.

cc @richardcase @justinsb