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 ?

Reproduce on Windows with Copilot CLI 1.0.51, then inspect the github-mcp-server startup and remote MCP connection reported by `/mcp show github-mcp-server`. Compare the behavior and logs with version 1.0.40, using the documented curl and Node.js fetch checks as controls. Done means the server connects successfully and `/mcp show` reports available tools instead of `fetch failed`.

索引モデルが issue の本文から書いたものです。