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)

Start by reproducing the `/mcp show` server-selection and `r` re-auth flow in Windows PowerShell with GitHub Copilot CLI 1.0.47, focusing on the browser account-selection step and the Streamable HTTP error. Compare the first failed attempt with subsequent retries. Done means the browser opens for every re-authentication attempt and a new account can be selected successfully.

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