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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