[Tests] Refactor Notifier tests to remove filesystem mocks

[gonzaloriestra]

Summary

Refactored packages/theme/src/cli/utilities/notifier.test.ts to remove fs/promises mocks in favor of using inTemporaryDirectory and real filesystem interactions.

Why

Following the "Tester" philosophy, real filesystem interactions in temporary directories are preferred over mocking node:fs to ensure tests are deterministic, more trustworthy, and better-aligned with actual behavior.

Changes

• Removed vi.mock('fs/promises') from packages/theme/src/cli/utilities/notifier.test.ts.
• Wrapped filesystem-dependent tests in inTemporaryDirectory.
• Replaced mock call assertions (expect(fs.writeFile).toHaveBeenCalledWith(...)) with actual file content assertions (expect(await readFile(path)).toEqual(...)).
• Updated the error path test to use a real directory path as an "invalid" file path to trigger a natural filesystem error.

Verification

• Ran pnpm --filter @shopify/theme vitest run src/cli/utilities/notifier.test.ts (All 7 tests passed).
• Ran pnpm --filter @shopify/theme lint (Passed).
• Ran pnpm --filter @shopify/theme type-check (Passed).
• Ran all unit tests in @shopify/theme package via pnpm --filter @shopify/theme vitest run (All 566 tests passed).

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PR created automatically by Jules for task 2324320246412902882 started by @gonzaloriestra

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