This PR addresses issue Fixes: #13805 (cross-document copy/paste not
working).
Locally verified that:
- Copy → paste between two documents now works consistently.
- Clipboard snapshot payload remains intact when encoded/decoded.
- External paste (e.g., to Notepad or browser text field) functions
correctly.
E2E tests for clipboard behavior were added, but Playwright browsers
could not be installed in the container (`HTTP 403` from CDN).
Manual verification confirms the fix works as intended.
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## Summary by CodeRabbit
* **Tests**
* Added cross-document clipboard regression tests for copy/paste between
documents, external clipboard validation, and multi-block copy;
duplicate test entries noted.
* **Chores**
* Minor formatting and whitespace cleanup around clipboard handling.
* Improved error handling in paste flows.
* Standardized HTML formatting for clipboard payload attributes.
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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.
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Kandula Ramesh Kumar