test: harden backend ref helpers per review
Commit
Soph2mo ago
- anyRefUnderPrefix now fails the test on a for-each-ref error instead of returning false, so a broken invocation can't make a "should NOT exist" assertion pass vacuously (matches RemoteCheckpointState).
- Drop the hand-rolled checkpointShard copy in e2e/testutil; reuse the production id.CheckpointID.ShardFor computation.
Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com Entire-Checkpoint: 08fd76379af0