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.

runtime/lua/vim/_defaults.lua

@@ -155,6 +155,30 @@ do
     end,
   })
 
+  vim.api.nvim_create_autocmd('TermRequest', {
+    group = nvim_terminal_augroup,
+    desc = 'Respond to OSC foreground/background color requests',
+    callback = function(args)
+      local fg_request = args.data == '\027]10;?'
+      local bg_request = args.data == '\027]11;?'
+      if fg_request or bg_request then
+        -- WARN: This does not return the actual foreground/background color,
+        -- but rather returns:
+        --   - fg=white/bg=black when Nvim option 'background' is 'dark'
+        --   - fg=black/bg=white when Nvim option 'background' is 'light'
+        local red, green, blue = 0, 0, 0
+        local bg_option_dark = vim.o.background == 'dark'
+        if (fg_request and bg_option_dark) or (bg_request and not bg_option_dark) then
+          red, green, blue = 65535, 65535, 65535
+        end
+        local command = fg_request and 10 or 11
+        local data = string.format('\027]%d;rgb:%04x/%04x/%04x\007', command, red, green, blue)
+        local channel = vim.bo[args.buf].channel
+        vim.api.nvim_chan_send(channel, data)
+      end
+    end,
+  })
+
   vim.api.nvim_create_autocmd('CmdwinEnter', {
     pattern = '[:>]',
     desc = 'Limit syntax sync to maxlines=1 in the command window',