Update: move test auth file in test GHA workflow for istio insecure overlay

[madmecodes]

Pull Request Template for Kubeflow Manifests

✏️ Summary of Changes

• Removed allow-namespace-traffic.yaml AuthorizationPolicy from the Istio insecure overlay
• Moved the AuthorizationPolicy creation to the GitHub Actions workflow (istio_validation.yaml)
• Added the policy creation step only for the 'insecure' test matrix mode with clear documentation explaining why it's needed

📦 Dependencies

https://github.com/kubeflow/manifests/pull/3153/files#r2154276621

🐛 Related Issues

none

✅ Contributor Checklist

• I have tested these changes with kustomize. See Installation Prerequisites.
• All commits are signed-off to satisfy the DCO check.
• I have considered adding my company to the adopters page to support Kubeflow and help the community, since I expect help from the community for my issue (see 1. and 2.).

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[juliusvonkohout]

Thank you
/lgtm
/approve

[google-oss-prow]

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