[Tests] Remove redundant filesystem mocks in stage-file.test.ts

[gonzaloriestra]

WHY are these changes introduced?

Technical debt cleanup. The test file packages/app/src/cli/services/bulk-operations/stage-file.test.ts contained redundant mocks for filesystem operations (readFile and fileSize) that are not used by the code under test, which instead uses Buffer.from and buffer.length.

WHAT is this pull request doing?

Removes vi.mock('@shopify/cli-kit/node/fs') and associated unused mock setups in packages/app/src/cli/services/bulk-operations/stage-file.test.ts.

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 18354233134098056568 started by @gonzaloriestra

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