MCP re-auth occasionally does not allow new account selection when authenticating

### Describe the bug

I tried authenticating to an MCP server via re-auth (`/mcp show` -> Selecting the server -> `r` for re-auth), and after following the browser-based auth flow (which should have worked) I got this error message in the CLI - `Streamable HTTP error: Error POSTing to endpoint:`

This is the first issue - would like to get some insight into why this is happening (happened at roughly 10:40 AM PST on 5/13/26). Secondly, now, even when I use "r", it's no longer letting me reauth to the triage MCP server. No browser pop up appears so the auth is stuck in a failed loop.

### Affected version

GitHub Copilot CLI 1.0.47

### Steps to reproduce the behavior

1. `/mcp show` -> Select a server -> `r` for re-auth
2. During the account selection flow, this error must be reproduced - "Streamable HTTP error: Error POSTing to endpoint:"
3. Try pressing `r` again for reauthenticating and observe that the browser no longer pops up

### Expected behavior

I would expect the browser to pop up every time a re-authentication is attempted

### Additional context

- Shell (Windows PowerShell)
- OS (Windows 11 Enterprise)

首先，在 Windows PowerShell 中使用 GitHub Copilot CLI 1.0.47 重現 `/mcp show` 的伺服器選擇和使用 `r` 重新驗證的流程，重點關注瀏覽器中的帳戶選擇步驟以及 Streamable HTTP 錯誤。比較第一次失敗的嘗試與後續重試。完成的標準是瀏覽器在每次重新驗證嘗試時都能開啟，並且可以成功選擇一個新帳戶。

由索引模型根據 Issue 內容生成。