feat(terminal): respond to OSC background and foreground request (#17197)

The motivation for this update is Issue #15365, where background=light
is not properly set for Nvim running from an Nvim :terminal. This can be
encountered when e.g., opening a terminal to make git commits, which
opens EDITOR=nvim in the nested terminal.

Under the implementation of this commit, the OSC response always
indicates a black or white foreground/background. While this may not
reflect the actual foreground/background color, it permits 'background'
to be retained for a nested Nvim instance running in the terminal
emulator. The behaviour matches Vim.

1 files changed, 13 insertions, 6 deletions

diff --git a/test/functional/core/startup_spec.lua b/test/functional/core/startup_spec.lua
index fd3748a985..f4a9c0c8d7 100644
--- a/test/functional/core/startup_spec.lua
+++ b/test/functional/core/startup_spec.lua
@@ -1369,12 +1369,19 @@ describe('inccommand on ex mode', function()
     local screen
     screen = Screen.new(60, 10)
     screen:attach()
-    local id = fn.termopen(
-      { nvim_prog, '-u', 'NONE', '-c', 'set termguicolors', '-E', 'test/README.md' },
-      {
-        env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
-      }
-    )
+    local id = fn.termopen({
+      nvim_prog,
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '-c',
+      'set termguicolors background=dark',
+      '-E',
+      'test/README.md',
+    }, {
+      env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
+    })
     fn.chansend(id, '%s/N')
     screen:expect {
       grid = [[