Ctrl-u cuts the line, but Ctrl-y doesn't put it back in Vi-mode

In Vi-mode, when using Ctrl+U to cut the current input it's impossible to use Ctrl+Y afterwards to get it back: it does nothing.

If I switch back to Emacs-mode and press Ctrl+Y, I get the data back. I can then switch back to Vi-mode.

I would expect Ctrl+Y to work the same way it works with readline, or for Ctrl+U to do nothing in Vi-mode (as the vi equivalent would be dd rather than Ctrl+U).

Version: v4.0.1

Start by reproducing the Ctrl-U and Ctrl-Y behavior in pgcli v4.0.1 while Vi-mode is enabled, then compare it with Emacs-mode and readline behavior. Trace the CLI's Vi-mode key handling and make the expected behavior consistent; verify the result by repeating both key sequences in each mode.

Written by the indexing model from the issue text.