Add undo to the AI assistant toolbar

I was a bit thrown off by the inability to discard/revert/undo the updated content, whenever running any of the controls. I know we have the main "Undo" control in the editor header, but it seems more appropriate to make a "Discard" control front and center in the block toolbar, which would omit anything the AI has done to/with my content.

I think this would encourage more experimenting as well, as you're much more confident to try it, if it's clear how to go back.

Start by locating the AI assistant block toolbar and the existing editor-header Undo control. Confirm how AI changes are tracked, then make the toolbar's discard action revert those changes and verify that the content returns to its prior state.

Written by the indexing model from the issue text.