Chinese Zhuyin IME preedit text accumulates in the lower-right corner on macOS

### Describe the bug

When using the macOS built-in Zhuyin input method in GitHub Copilot CLI on macOS, the IME composition/preedit text is rendered in the wrong place. Instead of appearing at the Copilot CLI input cursor, the composing Chinese text accumulates around the lower-right area of the terminal UI.

This appears related to the previously closed CJK IME positioning issue:
https://github.com/github/copilot-cli/issues/1698

That issue was reported for Japanese IME on Windows/WSL and was marked as fixed, but I can still reproduce a severe variant on macOS 26.5 with Copilot CLI 1.0.52.

User impact:

> 我每次輸入中文時，所有的文字都會累積在右下角，問題有點嚴重！

In practice, this makes Chinese input very difficult because the active composition text is detached from the actual prompt.

### Affected version

GitHub Copilot CLI 1.0.52

### Environment

- OS: macOS 26.5
- macOS build: 25F71
- Architecture: arm64
- Kernel: Darwin 25.5.0
- Copilot CLI: 1.0.52
- Copilot CLI path: `/opt/homebrew/bin/copilot`
- Terminal: Ghostty
- Input method: macOS built-in Zhuyin

### Steps to reproduce

1. Start GitHub Copilot CLI in interactive mode on macOS.
2. Switch to the macOS built-in Zhuyin input method.
3. Type Chinese text in the Copilot CLI prompt.
4. Observe where the IME composition/preedit text appears before committing the text.

### Expected behavior

The Chinese IME composition/preedit text and candidate UI should follow the active Copilot CLI input cursor, similar to native terminal input in shells such as zsh.

### Actual behavior

The Chinese composition/preedit text does not appear at the Copilot CLI input cursor. Instead, the text accumulates around the lower-right area of the terminal UI.

### Additional context

The previous issue's discussion suggested that the IME preedit may be rendered at the terminal's last write position instead of the actual input cursor position. The macOS behavior described here seems consistent with that kind of cursor-positioning problem.

This has only been observed in GitHub Copilot CLI so far. On the same machine and terminal/input-method setup, Codex CLI and Antigravity CLI 2.0 do not show this problem.

A screen recording below:

Start by reproducing the issue in interactive Copilot CLI on macOS with the built-in Zhuyin input method, then review the IME positioning discussion in issue #1698. Trace how the prompt's input cursor and terminal last-write position are handled for preedit text. Done means composition text and the candidate UI follow the active prompt cursor instead of accumulating in the lower-right.

索引モデルが issue の本文から書いたものです。