[Feat]: Update Documentation to Highlight Potential SSRF Risk

### Is your feature request related to a problem? Please describe.

Hello Team,

We recently observed a potential SSRF risk in the agent card endpoint which takes a user-supplied URL and fetches the JSON metadata from that URL.

If the endpoint is exposed without security controls on the Internet because the backend unconditionally fetches the user-supplied URL and allows the user to attach custom HTTP headers (or blindly forwards incoming headers) to the outbound request. This could enable, e.g., the user to fetch the GCP metadata endpoint.

Furthermore, the application's error handling is overly verbose: when it attempts to validate the fetched JSON against its expected schema (using Pydantic) and fails, it reflects the entire fetched payload back to the user in the error message.

### Describe the solution you'd like

Consider adding a warning in the README.md on the risk of hosting the tool unrestricted to the Internet and suggest users to rely on reverse proxy, authentication and outbound firewall rules to reduce the risk of abusing the functionality for SSRF.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Issue では README.md ファイルが示されています。既存のセキュリティに関する注記、またはデプロイに関するセクションを探してください。agent card endpoint を公開する際の SSRF リスクについて警告を追加し、緩和策として reverse proxy、認証、アウトバウンドのファイアウォールルールに言及してください。リポジトリ内の更新された README.md を確認して変更を検証してください。

索引モデルが issue の本文から書いたものです。