feat(security): enforce pod and container security contexts

[jsonmp-k8]

Summary

• Enforce default pod- and container-level security contexts across KFP v2 compiler templates (driver, executor, importer, DAG driver) and workflow spec.
• Harden control-plane manifests and GCP Marketplace chart with runAsNonRoot + RuntimeDefault seccomp, and container allowPrivilegeEscalation=false with dropped ALL capabilities.
• Update test workflows and compiled workflow goldens to include the new defaults.

Testing

• go test ./backend/src/v2/compiler/argocompiler/...
• go test ./backend/test/compiler/...

[github-actions]

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[google-oss-prow]

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[google-oss-prow]

Hi @jsonmp-k8. Thanks for your PR.

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