fix(tunnel): handle existing route on Windows gracefully

[VedantMadane]

Fixes #11645

On Windows, the route ADD command returns exit code 0 even when the route already exists. This causes minikube tunnel to infinitely retry adding the same route.

Fix by checking the output message for 'already exists' indication and treating it as success (making EnsureRouteIsAdded truly idempotent).

The issue manifests when running \minikube tunnel --cleanup\ on Windows - it gets stuck in a retry loop instead of completing successfully.

Testing

• Compiles with no errors
• Logic handles duplicate route additions gracefully
• Maintains backward compatibility with non-Windows platforms

[linux-foundation-easycla]

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[k8s-ci-robot]

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[k8s-ci-robot]

Hi @VedantMadane. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[minikube-bot]

Can one of the admins verify this patch?

[k8s-ci-robot]

PR needs rebase.

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