docs: add missing Box component properties and tests

[karan74jaiswal]

What this PR does:
This PR updates the Box component documentation to accurately reflect the available BoxOptions API in packages/core/src/renderables/Box.ts. It also adds behavioral test coverage for these previously undocumented properties.

While using opentui, I noticed that the online documentation for the Box component was missing several functional properties (such as single-sided borders, focus colors, and layout gaps).

Changes made:

• Docs: Added explicit code examples to box.mdx for custom border characters (BorderCharacters), single-sided borders (BorderSides), layout gaps (rowGap, columnGap), and focusable states.
• Docs: Formatted the properties table to perfectly match the alignment and strict TypeScript typings of other component documentation files (like input.mdx and text.mdx).
• Tests: Added comprehensive, use-case-driven tests in Box.test.ts for shouldFill, customBorderChars, focusedBorderColor, and flex gaps to ensure there are no regressions.

Testing:

• All 33 tests in Box.test.ts pass successfully.
• Ran bun run validate:docs, and the documentation structure, links, and code examples passed flawlessly.