/copy shows clipboard error but still copies successfully

### Describe the bug

When using the `/copy` command, the CLI displays an error:

```
✗ Failed to copy to clipboard: Error: Failed to create clipboard context:
$DISPLAY variable not set and no value was provided explicitly
```

However, the content **is** successfully copied to the clipboard (presumably
via OSC 52 fallback or similar terminal-native mechanism).

The error from the first clipboard backend attempt is surfaced to the user
even though the overall operation succeeds.

### Affected version

GitHub Copilot CLI 1.0.54

### Steps to reproduce the behavior

1. Run Copilot CLI inside a devcontainer (no X11/Wayland available)
2. Have a conversation with some output
3. Type `/copy`
4. Observe the error message (prefixed with ✗)
5. Paste elsewhere — the content was copied successfully

### Expected behavior

If the copy ultimately succeeds via a fallback backend, no error should be
shown. The error should only appear if ALL clipboard backends fail.

### Additional context

- Environment: DDEV-based devcontainer using
`ghcr.io/devcontainers/features/copilot-cli:1`
- No `$DISPLAY` is set (headless Docker container)
- Terminal: iTerm2 on macOS, connected to the devcontainer
- iTerm2 has "Applications in terminal may access clipboard" enabled
- iTerm2 supports OSC 52 clipboard escape sequences
- The ✗ prefix and "Failed" wording suggest a hard failure to the user,
but the operation actually succeeds

Start at the `/copy` command and its clipboard backend handling, then reproduce the behavior in a headless devcontainer with no `$DISPLAY`. Done means a successful fallback copy produces no error, while an error is shown only when all clipboard backends fail.

索引モデルが issue の本文から書いたものです。