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

[gonzaloriestra]

Description

This PR refactors packages/app/src/cli/services/build/bundle-size.test.ts to replace global filesystem mocks (vi.mock('@shopify/cli-kit/node/fs')) with real filesystem operations using inTemporaryDirectory and writeFile. This makes the tests more reliable and realistic by testing against the actual disk.

Changes

• Removed vi.mock('@shopify/cli-kit/node/fs').
• Wrapped test cases in inTemporaryDirectory.
• Used writeFile to create test files on disk.
• Updated assertions to use the actual file paths in the temporary directory.
• Cleaned up unused imports (vi, readFile).

Duplicate check

• Ran git log --grep="\[Tests\]" --oneline -n 100 and git log --grep="Jules" --oneline -n 100.
• Closest prior PR: [Tests] Remove filesystem mocks in execute-bulk-operation.test.ts.
• Why this is different: This PR targets a different service (bundle-size.test.ts) and addresses a different set of test cases related to bundle size calculation.

Verification

• Ran pnpm --filter @shopify/app vitest run src/cli/services/build/bundle-size.test.ts (all passed).
• Ran pnpm --filter @shopify/app lint (passed).
• Ran pnpm knip --workspace packages/app (passed).
• Ran pnpm --filter @shopify/app type-check (passed).
• Ran pnpm --filter @shopify/app vitest run (all 2188 tests passed).

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PR created automatically by Jules for task 6938643062223810454 started by @gonzaloriestra

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