Fix benchmark manifest Git SHA

[SPerekrestova]

Summary

• Pass the workflow commit SHA into the Cloud Run benchmark job as BENCHMARK_GIT_COMMIT.
• Make eval.run_benchmark use BENCHMARK_GIT_COMMIT/GITHUB_SHA when .git metadata is absent in the benchmark image.
• Tighten artifact validation so manifest.git_commit cannot be the all-zero fallback.

Test Plan

• 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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