gh-71880: Allow editing the last cell in curses.textpad.Textbox (GH-152363)

gh-71880: Allow editing the last cell in curses.textpad.Textbox (GH-152363)

Textbox.edit() ignored typing in the lower-right cell of the window.  It is
now written with insch(), which fills the cell without moving the cursor out
of the window (addch() there raises an error and scrolls a scrollable window).

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>