[Tests] Remove filesystem mocks in execute-command-helpers.test.ts

[gonzaloriestra]

WHY are these changes introduced?

This PR follows the testing protocol specified in .agents/automated-tasks/tests.md which mandates using real files and directories in temporary directories instead of mocking the filesystem.

WHAT is this pull request doing?

Refactored packages/app/src/cli/utilities/execute-command-helpers.test.ts to:

• Remove the global vi.mock('@shopify/cli-kit/node/fs').
• Replace all mocked usages of readFile and fileExists with real filesystem operations using inTemporaryDirectory and writeFile from @shopify/cli-kit/node/fs.
• Use joinPath from @shopify/cli-kit/node/path for cross-platform path handling.

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 9343488799109230646 started by @gonzaloriestra

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