Unable to authenticate Copilot

I am having the issue also reported here: https://github.com/orgs/community/discussions/126645

Setup: Copilot.vim is installed in git-for-windows VIM setup (cloned into `~/.vim/pack/github/start/copilot.vim`).
The browser is configured by `let g:browser = 'C:\Program Files\Mozilla Firefox\firefox.exe'`

After executing `:Copilot setup` I can authenticate using given OTP code, however on next try, copilot again wants to authenticate.
Also, in Qt creator, the sign in button is greyed.

Backporting to conpilot.vim version v1.11.3 makes it work: :Copilot setup gives me `Copilot: Authenticated as GitHub user (...)` and the Qt creator button turns active

使用 `:Copilot setup` 和已設定的 Firefox 瀏覽器，在 Git-for-Windows Vim 設定中重現該失敗，然後將行為與 copilot.vim v1.11.3 進行比較。檢查首次 OTP 流程後驗證是否仍然有效，以及 Qt Creator 的登入按鈕是否會變為啟用狀態；在受影響的設定中這兩種行為都能正常運作即表示完成。

由索引模型根據 Issue 內容生成。