Restore Ctrl-Y yank in Vi insert mode

[PNHD]

Summary

In Vi insert mode, Ctrl-U uses the Readline-style unix-line-discard binding, but Ctrl-Y is always consumed by the Vi completion handler even when no completion is active. This prevents yanking text killed by Ctrl-U.

This change keeps the existing Ctrl-Y completion behavior when a completion is active, and falls back to the Readline yank command otherwise. Application-defined Ctrl-Y bindings continue to override the default Vi binding.

This also addresses the behavior reported downstream in dbcli/pgcli#1608.

Testing

• regression tests for yank, active completion, and custom binding precedence
• full test suite on Python 3.10, 3.11, 3.12, 3.13, 3.14, and 3.14t
• mypy --strict for win32, linux, and darwin across the matrix
• Ruff check / format check and typos on Python 3.14