fix(sidebar): correctly exit visual mode

Commit a9b01e7 ("chores: use stopinsert (#1745)") replaced uses of
vim.api.nvim_feedkeys(...) with proper commands to exit and enter insert
mode. However "stopinsert" does not help when editor is in visual mode.

Unfortunately there is no dedicated command to exit visual mode so we
have to go back to simulating keystrokes again.

lua/avante/sidebar.lua

@@ -2726,8 +2726,10 @@ function Sidebar:create_input_container()
   place_sign_at_first_line(self.containers.input.bufnr)
 
   if Utils.in_visual_mode() then
-    -- Exit visual mode
-    vim.cmd("noautocmd stopinsert")
+    -- Exit visual mode. Unfortunately there is no appropriate command
+    -- so we have to simulate keystrokes.
+    local esc_key = api.nvim_replace_termcodes("<Esc>", true, false, true)
+    vim.api.nvim_feedkeys(esc_key, "n", false)
   end
 
   self:setup_window_navigation(self.containers.input)