[Tests] Remove filesystem mocks in bundle-ui-step.test.ts

[gonzaloriestra]

WHY are these changes introduced?

Filesystem mocks in tests prevent verifying actual filesystem side effects and can lead to implementation-coupled tests that don't catch real-world path handling issues.

WHAT is this pull request doing?

Refactors `packages/app/src/cli/services/build/steps/bundle-ui-step.test.ts` to replace global `vi.mock('@shopify/cli-kit/node/fs')` with real filesystem operations using `inTemporaryDirectory`. It uses `mkdir` and `writeFile` to set up the test environment and `fileExists` to verify the success of `copyFile`.

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 18014390488825622297 started by @gonzaloriestra

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