Enterprise accounts using github-mcp-server failed

### Describe the bug

When signed in to an enterprise account, tool calls using `github-mcp-server` uses the enterprise URL and fails for external lookups, but doenst look like it falls back to standard github url and just uses a standard web fetch instead of the MCP server

```
✗ Get file or directory contents (MCP: github-mcp-server) · docs/auth/index.md · owner: "github", repo: "copilot-sdk", path: "docs/auth/index.md"
└ MCP server 'github-mcp-server': failed to resolve git reference: failed to get repository info: GET https://api.ictcardiffcouncil.ghe.com/repos/github/copilot-sdk: 404 Not Found []

● Fetching web content https://raw.githubusercontent.com/github/copilot-sdk/main/docs/auth/index.md · max_length: 10000, raw: true, url: "https://raw.githubusercontent.com/github/copilot-…
└ Here is the raw content:
```

### Affected version

_No response_

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

在登录 enterprise 账户的情况下，使用 github-mcp-server 重现所示的 docs/auth/index.md 查找。跟踪 enterprise URL 的选择方式，以及外部仓库查找如何回退到标准 GitHub 路径。完成的标准是：查找不再在 enterprise URL 处失败，并使用预期的 MCP 或标准 fetch 行为。

由索引模型根据 Issue 内容生成。