[Tests] Remove filesystem mocks in dev-override.test.ts

[gonzaloriestra]

WHY are these changes introduced?

This PR refactors the unit tests for devWithOverrideFile to remove global filesystem mocking, following the codebase's best practices for more robust and realistic tests.

WHAT is this pull request doing?

• Removes vi.mock('@shopify/cli-kit/node/fs') from packages/theme/src/cli/services/dev-override.test.ts.
• Updates tests to use inTemporaryDirectory to perform real filesystem operations.
• Replaces manual vi.mocked calls for filesystem utilities with actual file creation using writeFile.

How to test your changes?

shopify theme preview

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 3785740012775383200 started by @gonzaloriestra

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