Optimize ShellV2 terminal input handling

[KCarretto]

This PR addresses a performance regression in the ShellV2 terminal where every keystroke triggered a full line redraw, causing input lag.

Changes:

• Modified useShellTerminal.ts to include a 'fast path' for common input scenarios (appending characters and backspacing at the end of the line).
• In the fast path, characters are written directly to the xterm.js instance, and a full redraw (for syntax highlighting) is debounced by 50ms.
• Complex edits (insertion/deletion in the middle of a line) continue to trigger an immediate redraw to ensure correctness.

Verification:

• Ran existing unit tests for shellUtils and useCallbackTimer (passed).
• Created a Playwright script (verify_shell.py) to simulate user input and verify the terminal UI renders correctly without errors.
• Visual verification via screenshot confirms the terminal prompt and input appear as expected.

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PR created automatically by Jules for task 8723481768795692919 started by @KCarretto

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