Revise DirectConnector branding and README to reflect current SDK architecture

The `DirectConnector` name is inherited from the early prototype that directly accessed API Hub before the Connectors SDK or AI Gateway existed. This branding is likely to cause confusion now.

Tasks:
- Rename the project folder and namespace from `DirectConnector` to something more descriptive (e.g., `ConnectorsSample`, `FunctionsConnectorSample`)
- Update the README content to better reflect the current SDK-based architecture
- Remove historical artifacts and terminology that reference the old direct-access approach
- Update the solution file project reference

首先定位 DirectConnector 项目文件夹、其 namespace 声明、README 以及解决方案文件中的项目引用。在选择替代名称之前，先审查仓库中现有的术语和所描述的 SDK 架构。文件夹、namespace、README、历史引用和解决方案引用都一致使用新的品牌名称后，即视为完成。

由索引模型根据 Issue 内容生成。