Add table menu actions to paste into cell or merge into table

What does this MR do and why?

Restores the ability to embed a copied table inside a single table cell in the rich text editor, which was lost with the multi-cell copy/paste fix in !249528 (merged). Since then, pasting copied cells into a table always merges them into the target table, and there was no way to nest a table inside a cell anymore.

This MR adds two actions to the table bubble menu ("Select action" dropdown):

• Paste into cell — inserts the clipboard content at the cursor, so a copied table becomes a nested table inside the cell (the previous behavior, on demand)
• Paste and merge into table — distributes the clipboard cells across the table rows/columns, auto-expanding the table if needed (same as the default Ctrl/Cmd+V behavior)

Keyboard paste behavior is unchanged: it keeps the merge behavior introduced in !249528 (merged).

Implementation details

• Menu actions run outside a paste event, so there is no event.clipboardData. The new commands read the clipboard via the async Clipboard API (navigator.clipboard.read()), which requires a secure context and prompts the user for clipboard permission on first use.
• pasteFromClipboardIntoCell feeds the clipboard HTML through the existing pasteContent path, so a <table> in the clipboard is inserted as a nested table at the cursor.
• pasteFromClipboardIntoTable reuses extractMultiCellTableHTML and delegates to prosemirror-tables' handlePaste (same code path as the keyboard paste); non-table clipboard content falls back to a regular paste.
• The two menu items are hidden when the Clipboard API is unavailable (canReadClipboard(): insecure context such as plain-HTTP instances, or browsers without clipboard.read() like Firefox < 127), so users are never shown an action that can only fail.
• Clipboard read failures (e.g. denied permission) emit the existing editor alert.

Considered alternative

A first iteration (commit e631abe0, still in this branch's history) showed a post-paste info alert offering to undo the merge and insert a nested table instead. It was replaced by the menu actions because they are persistent and discoverable rather than a transient alert, and they work before the paste happens instead of repairing it afterwards. Both approaches are shown in the recordings below.

References

• Resolves #627112 (closed)
• Regression introduced in !249528 (merged) (#585267 (closed))

Screenshots or screen recordings

Alternative 1: post-paste alert (not chosen)	Alternative 2: table menu actions (this MR)

How to set up and validate locally

Note

The async Clipboard API requires a secure context. On GDK over plain HTTP (http://gdk.test:3000), either enable HTTPS, or open chrome://flags/#unsafely-treat-insecure-origin-as-secure, add http://gdk.test:3000, and relaunch Chrome. Without this, the paste actions are hidden by design.

1. Open the rich text editor (for example, in an issue description or wiki page)
2. Insert a table and fill some cells
3. Select multiple cells (click a cell, then shift-click another) and copy (Cmd/Ctrl+C)
4. Click into a single cell of a table
5. Open the table's Select action dropdown at the table
6. Choose Paste into cell — the copied cells are inserted as a nested table inside the cell
7. Undo, then choose Paste and merge into table — the copied cells are distributed across the table
8. Verify Cmd/Ctrl+V still merges cells into the table as before
9. Verify that on a plain-HTTP origin (without the Chrome flag) the two paste actions do not appear in the dropdown

MR acceptance checklist

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