IME composition causes window flickering/shaking on Windows

## Description

When typing CJK characters (Chinese/Japanese/Korean) using Windows IME in the Copilot CLI, the terminal window visibly flickers/shakes on each keystroke during IME composition.

## Steps to Reproduce

1. Open Copilot CLI in Windows Terminal
2. Switch to a CJK input method (e.g., Microsoft Bopomofo / Pinyin)
3. Start typing Chinese characters
4. Observe the terminal window shaking/flickering as the IME candidate window appears

## Expected Behavior

The TUI should not re-render/redraw the entire frame during IME composition events. The input area should remain stable while the user is composing characters.

## Actual Behavior

Every IME composition event triggers a full TUI re-render, causing visible window flickering that makes the CLI uncomfortable to use with CJK input methods.

## Environment

- OS: Windows 11
- Terminal: Windows Terminal
- PowerShell: 7.6.1
- Copilot CLI: 1.0.40-0
- IME: Microsoft Bopomofo (Traditional Chinese)
- Code Page: 65001 (UTF-8)

## Impact

This significantly degrades UX for CJK users and discourages use of the CLI in non-English languages.

Beginne damit, das Problem in Copilot CLI in Windows Terminal mit der Eingabe über Microsoft Bopomofo oder Pinyin zu reproduzieren. Verfolge, wie IME-Kompositionsereignisse den TUI-Neuzeichnungspfad erreichen, und überprüfe, dass die Komposition keine vollständige Neuzeichnung des Frames mehr auslöst. Als abgeschlossen gilt die Aufgabe, wenn die CJK-Komposition visuell stabil bleibt und die normale Eingabe weiterhin funktioniert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.