fix: force benchmark job exit after upload

[SPerekrestova]

Summary

• add a guarded benchmark CLI exit path controlled by BENCHMARK_FORCE_OS_EXIT
• set the guard only on the Cloud Run benchmark job
• add tests for default SystemExit behavior and forced Cloud Run exit behavior

Why

After upgrading huggingface-hub, the Cloud Run smoke uploaded all artifacts but the process stayed alive after HF/Xet upload completion. The explicit Cloud Run guard lets the standalone benchmark worker exit cleanly after all work and flushes are done.

Validation

• rtk uv run python -m py_compile eval/run_benchmark.py
• rtk uv run pytest tests/eval/test_run_benchmark.py tests/test_tier1_benchmark_workflow.py -q
• rtk uv run pytest tests/eval tests/test_download_interaction_db.py tests/test_tier1_benchmark_workflow.py -q
• rtk uv run pytest tests/ --ignore=tests/test_rxnorm_client.py -q
• rtk git diff --check

[SPerekrestova]

LGTM

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Generated by Claude Code