[Tests] Add unit tests for deprecations store

[gonzaloriestra]

WHY are these changes introduced?

deprecations-store.ts in @shopify/cli-kit manages state for the earliest upcoming deprecation date across CLI calls, but previously lacked unit tests.

WHAT is this pull request doing?

Adds unit tests in packages/cli-kit/src/private/node/context/deprecations-store.test.ts covering getNextDeprecationDate and setNextDeprecationDate:

• Verifies default undefined state
• Handles empty date arrays and past dates
• Sets the earliest future deprecation date when provided multiple dates
• Updates stored date when a newer date is earlier, and preserves existing date when new dates are later

How to test your changes?

CI

Post-release steps

N/A

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 2854649748783195092 started by @gonzaloriestra

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