[Tests] Replace filesystem mock with real sparse file in bundle tests

[gonzaloriestra]

WHY are these changes introduced?

The current tests for app bundling mock the filesystem to simulate large file sizes. This is an anti-pattern that can lead to fragile tests and hides the actual behavior of the filesystem utilities. Using real temporary directories and sparse files provides more robust verification of bundle size limits.

WHAT is this pull request doing?

• Removes the module-level mock for @shopify/cli-kit/node/fs in packages/app/src/cli/services/bundle.test.ts.
• Updates the large bundle upload test to use truncate from node:fs/promises to create a real 101 MB sparse file.
• Ensures the test exercises the real fileSize utility from @shopify/cli-kit/node/fs.

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 8432853696511868224 started by @gonzaloriestra

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