[Tests] Add unit test coverage for systemTempDir

[gonzaloriestra]

WHY are these changes introduced?

Fixes #0000

The temp-dir.ts module in @shopify/cli-kit lacked unit test coverage to ensure systemTempDir is properly exported and points to a valid, existing system directory.

WHAT is this pull request doing?

Adds unit test coverage in packages/cli-kit/src/private/node/temp-dir.test.ts to verify that systemTempDir is exported as a non-empty string and resolves to an existing directory on the filesystem.

How to test your changes?

CI

Post-release steps

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 13153742942320172031 started by @gonzaloriestra

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