fix(editor): support copying single image from edgeless and pasting to page (#12709)

Closes: [BS-3586](https://linear.app/affine-design/issue/BS-3586/复制白板图片，然后粘贴到-page，图片失败)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **New Features**
  - Copying a single selected image in edgeless mode now places the image directly onto the system clipboard as a native image blob for smoother pasting.

- **Bug Fixes**
  - Enhanced clipboard handling to better manage image and text data inclusion, with improved fallback for snapshot HTML.

- **Tests**
  - Added an end-to-end test verifying image copy-paste functionality between edgeless and page editor modes.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/blocks/embed/src/embed-html-block/components/fullscreen-toolbar.ts

@@ -107,10 +107,10 @@ export class EmbedHtmlFullscreenToolbar extends LitElement {
     if (this._copied) return;
 
     this.embedHtml.std.clipboard
-      .writeToClipboard(items => {
-        items['text/plain'] = this.embedHtml.model.props.html ?? '';
-        return items;
-      })
+      .writeToClipboard(items => ({
+        ...items,
+        'text/plain': this.embedHtml.model.props.html ?? '',
+      }))
       .then(() => {
         this._copied = true;
         setTimeout(() => (this._copied = false), 1500);