🧪 test: add unit tests for deleteItemFromTree

[beingniloy]

This PR addresses the testing gap by adding comprehensive unit tests for the pure helper function deleteItemFromTree in src/utils.ts.

🎯 What

The testing gap for deleteItemFromTree has been addressed by introducing vitest unit tests.

📊 Coverage

The new test suite covers:

• Empty file tree.
• Root-level file deletion.
• Root-level folder deletion.
• Nested deep item deletion.
• Non-existent path behavior.
• Immutability and reference integrity (ensuring unchanged nodes keep their references, while modified node paths get cloned).

✨ Result

These tests ensure deleteItemFromTree works reliably and correctly handles the tree structure and immutability. All tests pass successfully under pnpm test.

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PR created automatically by Jules for task 7162340052108659191 started by @beingniloy

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