History navigation (↑/↓) scrolls line-by-line through multi-line prompts instead of jumping between entries

### Describe the bug

When navigating prompt history with `↑` / `↓`, multi-line prompts are traversed one line at a time rather than as a single entry. If a previous prompt is 30 lines long, pressing `↑` requires 30 keystrokes to pass through it and reach the entry before it. This makes history navigation for users who write long, detailed prompts effectively unusable.

### Affected version

GitHub Copilot CLI 0.0.420.

### Steps to reproduce the behavior

1. Submit a long multi-line prompt (10+ lines).
2. Submit another prompt after it.
3. Press `↑` in the empty input bar to navigate history.
4. Observe: the cursor moves through the long prompt one line at a time rather than jumping to it as a single unit.
5. You must press `↑` once per line of the previous prompt to navigate past it.

### Expected behavior

`↑` / `↓` in an empty input bar should jump between history **entries** as atomic units — the same behaviour as `bash`, `zsh`, `fish`, PowerShell, and every other shell. A 30-line prompt should be one `↑` press away, not 30.

When already inside a recalled multi-line prompt, `↑` / `↓` can navigate lines within it — but from an empty input, the first `↑` should always jump to the most recent entry in full.

### Additional context

- OS: Windows 11 Pro (Build 26200 / 24H2)
- Shell: PowerShell 7.5.4
- Copilot CLI: 0.0.420 (win32-x64)
- Workaround: use `Ctrl+G` to open the recalled prompt in an external editor — but this shouldn't be necessary for basic history navigation.

Reproduce the issue in the CLI input and history handling with a short prompt followed by a multi-line prompt, starting from an empty input bar. Verify that the first ↑ recalls the complete latest entry, while ↑/↓ inside a recalled entry can still move between its lines.

索引モデルが issue の本文から書いたものです。