refactor: replace Object.assign with spread syntax

[eatyourgreens]

Replace Object.assign with the much faster spread operator in the Classification model.

Please request review from @zooniverse/frontend team or an individual member of that team.

Package

lib-classifier

How to Review

No functional changes here. This modernises the classification code but doesn’t change how classifications work.

Checklist

PR Creator - Please cater the checklist to fit the review needed for your code changes.
PR Reviewer - Use the checklist during your review. Each point should be checkmarked or discussed before PR approval.

General

• Tests are passing locally and on Github
• Documentation is up to date and changelog has been updated if appropriate
• You can pnpm panic && pnpm bootstrap
• FEM works in all major desktop browsers: Firefox, Chrome, Edge, Safari (Use Browserstack account as needed)
• FEM works in a mobile browser

General UX

Example Staging Project: i-fancy-cats

• All pages of a FEM project load: Home Page, Classify Page, and About Pages
• Can submit a classification
• Can sign-in and sign-out
• The component is accessible
  • Can be used with a screen reader BBC guide to VoiceOver
  • Can be used from the keyboard WebAIM guide to keyboard testing
  • It is passing accessibility checks in the storybook

Refactoring

• The PR creator has described the reason for refactoring.
• The refactored component(s) continue to work as expected.

[Copilot]

Pull request overview

This PR modernizes the Classification mobx-state-tree model snapshot handling by replacing Object.assign usage with object spread syntax, keeping snapshot copying semantics the same while simplifying the code.

Changes:

• Update toSnapshot() to return a spread-copied snapshot with annotations overridden as an array.
• Replace Object.assign({}, snapshot) with { ...snapshot } in preProcessSnapshot and postProcessSnapshot.

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