fix(table): paste copied table cells into target cells

[mejo-]

Pasting copied table cells into a table cell was refused with a "nested tables are not supported" warning.

Cells and rows copied from a ProseMirror editor (marked with data-pm-slice) now keep their type when parsed. Pasted header cells and rows are converted to body ones. A single copied cell is unwrapped and its content pasted at the cursor. Several cells are handed to the prosemirror-tables paste handler, which writes them into the target cells from the cursor onwards and adds rows and columns as needed. Tables pasted from spreadsheet apps go the same way, instead of keeping only the first cell's content.

Pasting a table together with other content is still rejected as a nested table.

Assisted-by: ClaudeCode:claude-opus-5-5

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• Code is properly formatted (npm run lint / npm run stylelint / composer run cs:check)
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[mejo-]

/backport to stable35

[benjaminfrueh]

Hi @mejo- and thanks for this! I was testing this and found a possible regression:

Copying text from inside a table cell and pasting it into another cell no longer works and is rejected with the table was pasted into a table error. On main the text is pasted but as a new line in the cell instead of inline at the cursor.

The single cell case at line 110 would handle this, but it is never reached. For text copied from inside a cell, pastedCells() doesn't find a cell unlike when whole cells are copied. Maybe the pasted content could be adjusted so that pastedCells() finds the single cell here.

[samin-z]

i also found this, if one cell has content with a bullet at the end, if we copy the first cell and paste it in the beginning of another cell with existing content, the second cell's content becomes part of the bullet.
this is because one whole cell is pasted, 'Slice.maxOpen()' opens the paste into the bullet so the text that was already in the cell gets pulled into it. previous code left that text after the list.

[mejo-]

Thanks @benjaminfrueh and @samin-z. Both good catches! Indeed I only tested pasting full copied cells, not selected content from another cell. I pushed a fix for both findings.

[benjaminfrueh]

Nice! Looks good to me now 🚀