[BUG] 症状: チャット入力欄で日本語IME変換中に下線・色分けが表示されない

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

チャット入力欄で日本語IME変換中に下線・色分けが表示されない。環境: Windows 11、Microsoft IME、VSCode拡張版のClaude Code 比較結果: VSCodeの通常のエディタ画面では下線が正常に表示される

### What Should Happen?

Microsoft IMEと同じ環境で入力できる

### Error Messages/Logs

```shell

```

### Steps to Reproduce

VSCodeでClaude Code拡張機能のチャット入力欄を開く
Microsoft IMEで日本語入力をオンにする
ひらがなを入力し、変換前の状態にする(例:「にゅうりょく」と入力)
スペースキーで変換する
期待される動作:変換中の文字に下線が表示され、確定前の文字と分かる

実際の動作:下線や色分けが表示されず、変換中か確定済みかが見た目で区別できない

補足:同じ操作をVSCodeの通常のエディタ(ファイル編集画面)で行うと、下線は正常に表示される

### Claude Model

Sonnet (default)

### Is this a regression?

I don't know

### Last Working Version

不明

### Claude Code Version

2.1.263(Windows 64bit版)

### Platform

Anthropic API

### Operating System

Windows

### Terminal/Shell

VS Code integrated terminal

### Additional Information

_No response_

Start by reproducing in the VSCode Claude Code extension chat input on Windows 11 with Microsoft IME, then compare the same IME flow in a normal VSCode editor buffer. Read the chat input implementation and any composition/IME event handling around it. Done means unconfirmed Japanese IME text is visually distinguishable with underline or coloring during conversion, as in the VSCode editor.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.