patch 9.1.0788: <CSI>27;<mod>u is not decoded to literal Escape in kitty/foot

Problem:  <CSI>27;<mod>u is not decoded to literal Escape in kitty/foot
Solution: disable XTerm modifyOtherKeys form 1 when the kitty protocol is enabled
          (Christian Fillion)

References:
- https://invisible-island.net/xterm/modified-keys.html
- https://sw.kovidgoyal.net/kitty/keyboard-protocol/
- https://codeberg.org/dnkl/foot/src/commit/e891abdd6a6652bd46b28c1988700a7f30931210/kitty-keymap.h
- https://github.com/kovidgoyal/kitty/blob/d31459b0926f2afddc317d76314e4afd0d07d473/kitty/key_encoding.c#L193

fixes: #15868
closes: #15881

Signed-off-by: Christian Fillion <contact@cfillion.ca>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_termcodes.vim b/src/testdir/test_termcodes.vim
index 507753c..2b44b15 100644
--- a/src/testdir/test_termcodes.vim
+++ b/src/testdir/test_termcodes.vim
@@ -2001,6 +2001,12 @@
         \ kitty: 'y',
         \ }, terminalprops())
 
+  call feedkeys("\<Esc>[?1u") " simulate the kitty keyboard protocol is enabled
+  call feedkeys(':' .. GetEscCodeCSIu('V', '5') .. GetEscCodeCSIuWithoutModifier("\<Esc>") .. "\<C-B>\"\<CR>", 'Lx!')
+  call assert_equal("\"\<Esc>", @:)
+  call feedkeys(':' .. GetEscCodeCSIu('V', '5') .. GetEscCodeCSIu("\<Esc>", '129') .. "\<C-B>\"\<CR>", 'Lx!')
+  call assert_equal("\"\<Esc>", @:)
+
   set t_RV=
   call test_override('term_props', 0)
 endfunc