fix : Code widget renders Unicode pipe character without ASCII fallback

[tmdeveloper007]

Summary of What Needs to be Done

The Code widget (packages/widgets/src/display/Code.ts) renders a vertical pipe character (│) as the separator between line numbers and code content without checking caps.unicode. When the terminal does not support Unicode/box-drawing characters, this renders incorrectly.

Changes that Need to be Made

1. In packages/widgets/src/display/Code.ts, add a caps.unicode check before rendering the vertical separator character in _renderSelf().
2. Use caps.unicode ? '│' : '|' for the separator character.
3. Add tests that verify the ASCII fallback renders correctly when caps.unicode is false.

Impact that it would Provide

The Code widget will render correctly in terminals that do not support Unicode box-drawing characters (e.g., some legacy terminals, certain CI environments, or terminals with TERM=dumb).

Note: Please assign this issue to the tmdeveloper007 account.

[github-actions]

✅ Auto-assigned to @tmdeveloper007 (issue opener).

Open your PR by 2026-08-10 (7 days). The bot frees this issue for another contributor after that date.

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• PR title: type: short description
• Link this issue in your PR body: Closes #3430

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