Add tests for clipboard image functionality and mesh plane asset creation

- Implemented a comprehensive test suite for the clipboard image module, covering various functionalities including operator registration, context handling, and image processing.
- Created a new mesh plane asset with customizable parameters and integrated it into the geometry node system.
- Developed a validation script to ensure the correct behavior of the mesh plane asset, including input handling and output verification.

docs/remove-background.md

@@ -34,3 +34,5 @@ Depth Plane 和 Surface 的转换对话框也提供 **Remove Background**：
 ### 透明边缘需要调整
 
 当前正式入口不提供 BEN2 refinement 参数。可以在结果材质中继续处理 Alpha，或在外部图像软件中修整生成的颜色图。
+
+输入图片已有 Alpha 时，AnyImage 会先用该 Alpha 遮住 BEN2 推理输入中的透明 RGB，再将结果 Alpha 计算为“输入 Alpha × BEN2 Alpha”。因此 Image Box / Lasso 或外部工具创建的透明范围不会被去背景重新扩张；软边缘也会继续保留。