IME input issue: Enter key sends message during Japanese text conversion

Description

When using Japanese IME (Input Method Editor), pressing Enter to confirm text conversion simultaneously sends the message, making it very difficult to use.

Steps to Reproduce

1. Enable Japanese IME
2. Start typing in Japanese (e.g., type "henkan" in romaji)
3. Press Enter to confirm the conversion to Japanese characters
4. Expected: Text conversion should be confirmed
5. Actual: Text conversion is confirmed AND the message is immediately sent

Expected Behavior

The Enter key should only confirm IME conversion when composing text. A separate action (like Cmd/Ctrl+Enter or clicking Send button) should be required to send the message.

Environment

• OS: macOS (Darwin 24.6.0)
• VSCode version: 1.104.2
• Claude Code version: 2.0.2
• Language: Japanese IME

Additional Context

This is a common issue with text input fields that don't properly handle IME composition events. Many chat applications (Slack, Discord, etc.) solve this by distinguishing between IME composition events and regular Enter key presses.