Test: added unit tests for Box component

[imayuss]

Short description of the fix

Added unit tests for the Box component covering children rendering, border/ padding/ style props & edge cases.

• Added box.test.tsx following the block.test.tsx conventions.
• Covered default rendering, custom/ nested children, empty children, hidden, default & custom padding , each border prop individually plus all-on/ all-off combinations, default borderRadius & boxSizing: border-box, removeDefaultStyle.
• Covered custom style merging/ overriding, a darkMode smoke test, very large (5000 char) content & an optional snapshot.

Files changed: tests/ui/basic-components/box.test.tsx

Issue to close #69

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• Synced repo before changes?
• Documentation updated (if needed)?
• Functionality verified locally?