test: pin leo-sdk-nodejs behavior as the reference for rstreams-rs parity

[mecham-lynn]

Summary

Tests only, with no source changes. This pins leo-sdk-nodejs's behavior as the reference that rstreams-rs (Rust) is being brought to parity with.

• New test/*.utest.ts (sinon + chai, in the style of test/lib.cron.utest.ts) cover:
  • the config provider chain and its resolution order
  • cron: lock, checkpoint, run-again, schedule, trigger, report-complete
  • write options on toLeo (Kinesis/Firehose/S3, partitioning, backoff, toCheckpoint)
  • fast_s3_read retries and read hooks
  • eid helpers
  • the fanout wrapper
  • dynamodb.js
  • event-trace
• child_process.fork is stubbed in the fanout tests. Outside Lambda, wrappers/fanout.js forks process.argv[1], which under mocha re-runs mocha.
• AGENTS.md and CLAUDE.md added for contributors and agents.

Evidence

• 301 → 685 passing tests.

• Line coverage on the files the Rust port is checked against:

  File	Lines covered
  lib/cron.js	99.5%
  rstreams-config-provider-chain	97.5%
  wrappers/fanout.js	93.8%
  lib/dynamodb.js	89.0%
  lib/stream/leo-stream.js	88.8%
  event-trace	82.9%

• Deliberate breaks of the lock condition, the stale-checkpoint condition, and config secret order are each caught by a failing test.

• Found while writing the tests and recorded rather than changed, since this PR changes no source: a rejected combined Firehose put is retried as the first record only (leo-stream.js:876-896), so the rest of that payload is dropped.

Testing

npm ci
npx tsc --watch false
npm run coverage-all

Companion PR in Chub-Engineering/rstreams-rs uses these behaviors as its oracle.

Task: TASK-002 (no Jira yet)

🤖 Generated with Claude Code

---

Note

Low Risk
Tests, documentation, and CI action pinning only; runtime SDK behavior is unchanged.

Overview
Adds a large behavioral test suite (TypeScript test/*.utest.ts, run as compiled *.utest.js) to lock in leo-sdk-nodejs as the reference for rstreams-rs parity. Coverage spans config resolution order, cron lifecycle, DynamoDB helpers, leo-stream read/write paths (including fast_s3_read), event IDs, event-trace, and the fanout wrapper—with DynamoDB/AWS clients stubbed via sinon.

Also adds AGENTS.md (build/test conventions, gotchas) and CLAUDE.md (alias to AGENTS.md). CI pins actions/checkout, setup-node, and upload-artifact to specific commit SHAs instead of floating @v3/@v4 tags.

No production library code changes.

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