[Refactor] modernize errorHandler and improve test coverage

[gonzaloriestra]

WHY are these changes introduced?

The errorHandler function in @shopify/cli-kit used an older Promise-based pattern with .then() chains, which was less readable and more difficult to maintain than modern async/await. Additionally, the AbortSilentError branch was not explicitly verified by unit tests.

WHAT is this pull request doing?

• Refactored errorHandler in packages/cli-kit/src/public/node/error-handler.ts to use async/await syntax.
• Flattened the logic using early returns for CancelExecution and AbortSilentError, reducing nesting.
• Added a new unit test in packages/cli-kit/src/public/node/error-handler.test.ts to verify that AbortSilentError is handled silently without exiting or logging.
• Improved test isolation in the new test case by explicitly clearing captured output.

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 11091565751469337296 started by @gonzaloriestra

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