Fix missing errors when rejecting execution

[jpwilliams]

Summary

Fixes error sometimes being undefined when a step rejects when using the InngestTestEngine, caused by not always looking for the correct value from the returned checkpoints.

Checklist

• [ ] ~Added a docs PR that references this PR~ N/A Bug fix
• [ ] ~Added unit/integration tests~ N/A
• [x] Added changesets if applicable

Related

• Closes https://github.com/inngest/inngest-js/issues/732#issuecomment-2484136903

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: bb5c783eb0e36c0335cf9652f6c06f78dde6b1e7

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------------- | ----- | | @inngest/test | Patch |

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