[Tests] Add unit tests for groupBy and partition

[gonzaloriestra]

WHY are these changes introduced?

Fixes #0000

Increases test coverage and confidence for collection utility functions (groupBy and partition) in @shopify/cli-kit.

WHAT is this pull request doing?

Adds unit tests in packages/cli-kit/src/public/common/collection.test.ts covering:

• groupBy with custom function iteratees.
• groupBy handling of empty array, null, and undefined inputs.
• partition handling of empty array, null, and undefined inputs.

How to test your changes?

CI

Post-release steps

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

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PR created automatically by Jules for task 16635636750787613929 started by @gonzaloriestra

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