[Refactor] Modernize errorHandler in cli-kit

[gonzaloriestra]

WHY are these changes introduced?

The errorHandler function in @shopify/cli-kit used a nested .then() promise chain and complex if/else if/else nesting, which made it harder to read and maintain than modern async/await patterns.

WHAT is this pull request doing?

• Refactors errorHandler in packages/cli-kit/src/public/node/error-handler.ts to use async/await.
• Simplifies the control flow using early returns to flatten the function structure.
• Adds a unit test in packages/cli-kit/src/public/node/error-handler.test.ts to verify generic error handling.
• Fixes the @oclif/core mock in tests to preserve original exports while overriding settings.

How to test your changes?

CI

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 5284855694113462077 started by @gonzaloriestra

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