clusterapi: fall back to replica decrement for MachinePool nodes without matching Machine

[mnaissiou]

What type of PR is this?

/kind bug

What this PR does / why we need it:

This PR fixes the scale-down behavior for Cluster API MachinePool-backed node groups.

The current deletion flow attempts to find a matching Machine object for the node being removed. However, for MachinePool-backed nodes, relying on the presence of an individual Machine is not always appropriate. In this case, scale-down should still proceed by decreasing the MachinePool replica count.

This change makes the MachinePool deletion path fall back to replica decrement when no matching Machine is found, instead of failing the scale-down operation.

This PR adds that fallback so MachinePool scale-down does not fail when no matching Machine is found.

Which issue(s) this PR fixes:

Fixes #9681

Special notes for your reviewer:

This change is limited to the MachinePool scale-down path.
If a matching Machine is found, the existing behavior is preserved.
If no matching Machine is found, the implementation falls back to decrementing the MachinePool replicas count.

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mnaissiou
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