[Tests] Remove filesystem mocks in result.test.ts

[gonzaloriestra]

This PR refactors the unit tests for writeOrOutputStoreExecuteResult in the @shopify/store package to eliminate filesystem mocks. Following the project's testing strategy, it now uses real filesystem operations within temporary directories provided by inTemporaryDirectory.

Key changes:

• Removed vi.mock('@shopify/cli-kit/node/fs').
• Updated tests to use inTemporaryDirectory and joinPath for managing test files.
• Replaced expect(writeFile).toHaveBeenCalledWith(...) assertions with expect(await readFile(outputPath)).toContain(...).
• Verified that all tests pass and that linting/type-checking/knip are clean.

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PR created automatically by Jules for task 9552238169850303503 started by @gonzaloriestra

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