Fix fetch assignability tests for Node 20

[jpwilliams]

Summary

Fixes some tests that have started failing due to readonly assignability.

Checklist

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

[changeset-bot[bot]]

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Latest commit: 71e7ba2b01b6d19c4a544d0e6171fd308bfbb8eb

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