Validate managed EKS node-group creation before graduation

> 🤖 Generated by the Agentic Engineer

## Outcome

Validate the managed EKS node-group addition path from #6921 on a disposable, explicitly authorized cluster before removing `spec.cluster.eks.experimentalManagedNodegroupCreation`.

Part of #4328. Blocked by #6921. The option stays off by default until this evidence is complete.

## Acceptance criteria

- Record the exact KSail revision, eksctl version, AWS region, and sanitized configuration used for the evaluation. Keep account identifiers and credentials out of public evidence.
- Prove the default-off behavior, then opt in and add a managed group with declared networking, labels, capacity, and IAM settings. Verify the cluster's original groups remain intact and the new group becomes ready.
- Verify the SDK inventory covers every EKS page and refuses failed or incomplete EKS/CloudFormation reads. Confirm an unmanaged or unfinished same-name stack prevents creation.
- Exercise source drift, ownership refusal, cancellation, a failed second addition, a successful retry, and a concurrent same-name create. Verify that only this invocation's creation marker authorizes reporting an addition as applied.
- Repeat an unchanged update and prove no group is recreated or duplicated. Exercise scaling with autoscaler-owned desired capacity after a slow addition.
- Delete the disposable resources and verify cleanup. Document exact evidence and remaining limits before a separate graduation PR removes the experimental gate.

This issue authorizes no cloud provisioning by itself. The evaluation needs a maintainer-approved disposable target and cleanup plan.

Start with the managed EKS node-group addition path from #6921 and the experimentalManagedNodegroupCreation option. Proceed only after maintainer approval of a disposable target and cleanup plan; done means the listed creation, inventory, concurrency, retry, scaling, unchanged-update, cleanup, and evidence checks pass without exposing credentials.

Written by the indexing model from the issue text.