[Tests] Refactor asset-ignore tests to use real filesystem

[gonzaloriestra]

Refactored packages/theme/src/cli/utilities/asset-ignore.test.ts to replace filesystem mocks with real operations using inTemporaryDirectory and writeFile. This makes the tests more robust and aligns with the project's testing standards.

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PR created automatically by Jules for task 8912396214291889262 started by @gonzaloriestra

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