Add focused regression coverage for validation rules called out in PLAN.md.
Chunk upload tests now prove that an otherwise valid upload rejects an
out-of-range chunk index through the HTTP API. Completion tests now prove that a
manually corrupted chunk file is not assembled into a final file.
Update TESTS.md so the reusable checklist reflects these automated proofs.
Test Plan:
- just check
- just nginx-smoke
Refs: PLAN.md validation checklist
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