github-mcp-server fails with "fetch failed" since 1.0.51 update

### Describe the bug

Since updating to 1.0.51 the built-in github-mcp-server fails to connect on Windows with "fetch failed". This was not present in any previous version. Logs confirm no github-mcp-server connection attempts existed before 1.0.51.

### Affected version

GitHub Copilot CLI 1.0.51.

### Steps to reproduce the behavior

1. Run copilot on Windows with version 1.0.51
2. Observe "Failed to connect to MCP server 'github-mcp-server'" on startup
3. Run /mcp show github-mcp-server
4. Observe:

MCP Server: github-mcp-server
Type: http
URL: https://api.business.githubcopilot.com/mcp/readonly
Status: ✗ Failed
Error: fetch failed
Tools: Server failed to connect

Log shows:
2026-05-21T15:42:09.074Z [ERROR] Starting remote MCP client for github-mcp-server with url: https://api.business.githubcopilot.com/mcp/readonly
2026-05-21T15:42:09.456Z [ERROR] MCP transport for github-mcp-server closed
2026-05-21T15:42:09.457Z [ERROR] Failed to start MCP client for remote server github-mcp-server: TypeError: fetch failed

### Expected behavior

github-mcp-server connects successfully

### Additional context

- curl to https://api.business.githubcopilot.com/mcp/readonly returns 401 (expected, endpoint reachable)
- node fetch to same endpoint returns 401 (expected, Node.js networking works)
- node --use-system-ca fetch returns 401 (expected, no SSL inspection issue)
- Fresh login via `copilot login` performed, credential confirmed in Windows Credential Manager
- No corporate proxy environment variables set
- Previous logs (1.0.40, May 12-20) show no github-mcp-server connection attempts at all
- Issue began when 1.0.51 binary was installed

Windows (win32-x64)
Node.js v24.14.0

Potential regression for #2282 ?

Reproduire sous Windows avec Copilot CLI 1.0.51, puis inspecter le démarrage de github-mcp-server et la connexion MCP distante signalée par `/mcp show github-mcp-server`. Comparer le comportement et les logs avec la version 1.0.40, en utilisant comme contrôles les vérifications documentées avec curl et Node.js fetch. La tâche est terminée lorsque le serveur se connecte correctement et que `/mcp show` signale les outils disponibles au lieu de `fetch failed`.

Rédigé par le modèle d'indexation à partir du texte de l'issue.