🐛 Fix kubeconfig flakes

[ntnn]

My understanding of the Eventually is that it returns the first time the wrapped Gomega doesn't error.
That means that these tests race the reconciler setting the stomach key to food, so sometimes they can return empty string which then fails the test because the returned value is just empty string.

This moves all of the .Shoudl(Equal( tests inside of the Eventually so these tests don't flake anymore.

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[ntnn]

An example of a failing run: https://github.com/kubernetes-sigs/multicluster-runtime/actions/runs/19634022707/job/56220282989?pr=108

   [FAILED] Timed out after 10.001s.
  Expected
      <string>: 
  to equal
      <string>: food
  In [It] at: /home/runner/work/multicluster-runtime/multicluster-runtime/providers/kubeconfig/provider_test.go:263 @ 11/24/25 12:21:20.198

[ntnn]

Closing, I don't think this will actually fix the flakes. I think the problem is in the provider itself.