[CI][AMD] Add Hugging Face offline retry policy

[AndreasKaratzas]

• add a typed, AMD-only hf_offline_retry policy to the Python pipeline generator
• emit an authoritative retry flag for eligible native and Docker-in-Docker AMD jobs
• keep the AMD Jinja path aligned, with explicit opt-outs and fail-closed validation
• remove only Buildkite's exit-status-1 retry when the offline/online wrapper owns that retry
• document the policy and add Python-generator and Jinja-template regression tests

This is the ci-infra half of an AMD-scoped pilot. Hugging Face-backed tests should run from cache in offline mode first, then retry once online only for eligible test failures. Keeping the pilot limited to AMD jobs lets us validate the behavior before considering wider rollout.

Companion PR

• vLLM runner/helper implementation: [CI][AMD] Retry Hugging Face tests online vllm#49518