Support for ${input:name} in mcp.json

### Describe the bug

Now that the cli supports loading mcp server configuration from `.vscode/mcp.json` it's a problem it does not support asking for inputs. It makes the feature kind of useless because now I have to commit my secrets if I want to share the server configurations with my colleagues.

### Affected version

GitHub Copilot CLI 0.0.421. Run 'copilot update' to check for updates.

### Steps to reproduce the behavior

Start copilot in a folder with a mcp-json with ${input:____}.
Example:

```
{
"servers": {
"context7": {
"type": "http",
"url": "https://mcp.context7.com/mcp",
"headers": {
"CONTEXT7_API_KEY": "${input:context7_api_key}"
}
},
},
"inputs": [
{
"id": "context7_api_key",
"type": "promptString",
"description": "api key",
"password": true
}
]
}
```

### Expected behavior

Ask the user for input or look for a separate file containing secrets

### Additional context

_No response_

先閱讀 `.vscode/mcp.json` 中的設定範例，以及本 issue 中描述的 `inputs` 區段。追蹤 CLI 如何載入 MCP 伺服器設定，並確認應如何取得 `${input:name}` 值，包括密碼輸入或獨立的 secrets 檔案；當共用設定不再需要提交 secrets 時，即表示完成。

由索引模型根據 Issue 內容生成。