fix(openai): add backward compat for get_user_message (#813)

Co-authored-by: ming.chen <ming.chen@shopee.com>
Co-authored-by: Aaron Pham <Aaronpham0103@gmail.com>

lua/avante/providers/openai.lua

@@ -39,7 +39,17 @@ M.role_map = {
 }
 
 ---@param opts AvantePromptOptions
-M.get_user_message = function(opts) return table.concat(opts.messages, "\n") end
+M.get_user_message = function(opts)
+  vim.deprecate("get_user_message", "parse_messages", "0.1.0", "avante.nvim")
+  return table.concat(
+    vim.iter(opts.messages):filter(function(_, value) return value.role == "user" end):fold({}, function(acc, value)
+      acc = vim.list_extend({}, acc)
+      acc = vim.list_extend(acc, { value.content })
+      return acc
+    end),
+    "\n"
+  )
+end
 
 M.parse_messages = function(opts)
   local messages = {}