fcitx5 preedit (input preview) not visible in Gnome Terminal

## Problem

When using Copilot CLI in Gnome Terminal on Ubuntu with fcitx5 input method, the Chinese character preview (preedit) is not visible during input. This affects all fcitx5 input methods including Japanese IME.

## Environment

- **OS**: Ubuntu with Gnome Terminal
- **Input Method**: fcitx5 (tested with fcitx5-mcbopomofo and Japanese IME)
- **Copilot CLI**: Latest version
- **Terminal**: Gnome Terminal

## Current Behavior

When typing Chinese or Japanese characters using fcitx5:
- The preedit text (composing buffer) does not appear in the terminal
- Users cannot see what they are typing until they confirm the input
- This makes input method usage nearly impossible

## Root Cause

According to fcitx5 maintainer feedback (see [fcitx5-mcbopomofo#226](https://github.com/openvanilla/fcitx5-mcbopomofo/pull/226)), Copilot CLI's TTY control causes Gnome Terminal's preedit display to disappear. This affects all fcitx5 input methods, not just Chinese.

## Current Workaround

Disabling "Show preedit in application" in fcitx5 settings forces a floating window to appear, making input visible. However, this degrades the user experience across **all** GUI applications, not just the terminal.

## Expected Behavior

fcitx5 preedit should be visible in Copilot CLI, just as it works in other terminal applications like bash, zsh, or vim.

## Additional Context

This issue was initially investigated as a potential fcitx5 bug, but the maintainer confirmed it's related to how Copilot CLI controls the TTY. Quote from maintainer:

> "Copilot CLI does cause Gnome Terminal's own preedit to disappear. Other fcitx5's input methods, such as the officially packaged Japanese IME, are also affected. This all points to some issues with Copilot CLI."

Reference: https://github.com/openvanilla/fcitx5-mcbopomofo/pull/226#issuecomment-3622987241