Draft: Fix multi-cell copy/paste in rich text editor tables

What does this MR do and why?

Fix multi-cell copy/paste functionality in GitLab's rich text editor tables.

When copying multiple table cells and pasting into a table in the rich text editor, the content was being pasted as markdown into a single cell instead of being distributed across the appropriate cells.

Root cause: The copyPaste extension (with EXTENSION_PRIORITY_HIGHEST) intercepted all paste events before the prosemirror-tables handlePaste could run. The prosemirror-tables handlePaste correctly distributes pasted cells across rows and columns and auto-expands the table when needed.

Fix: When the user is pasting inside a table and the clipboard contains HTML with table cells, the paste event is delegated to prosemirror-tables' handlePaste, which handles:

• Distributing cell content across the correct rows and columns
• Auto-expanding the table when pasting more cells than existing columns

References

Closes #585267 (closed)

Screenshots or screen recordings

Before	After
Pasting multiple cells inserts all content as markdown into a single cell	Pasting multiple cells distributes content across the correct cells, auto-expanding the table if needed

How to set up and validate locally

1. Open the rich text editor (e.g., in a Wiki page or issue description)
2. Create a table with multiple rows and columns
3. Select multiple cells (e.g., a column of values)
4. Copy the selection (Cmd/Ctrl+C)
5. Click on a cell in another table (or the same table)
6. Paste (Cmd/Ctrl+V)
7. Verify that the pasted content is distributed across the correct cells instead of being inserted as markdown into a single cell

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.