test: address Copilot review on infra helpers
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- AssertCheckpointsOnRemote (git-refs arm): surface git errors instead of reporting a misleading "found none" via gitOutputSafe
- RemoteCheckpointState: fail the test on git error instead of returning "" (two broken invocations comparing equal would pass vacuously)
- hermeticity self-test comment: the guard is per-host http proxy entries, not an insteadOf redirect
Co-Authored-By: Claude Fable 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_012yi3hHGAGepwfrfjPETjGq Entire-Checkpoint: d4f06e259d79
Checkpoints
Checkpoint 1