refactor(message): add helper to update contents of simple text message

Add update_content() helper so code outside of history module does not
need to know details of the message structure.

lua/avante/history/message.lua

@@ -53,4 +53,11 @@ function M:new_assistant_synthetic(item) return M:new_synthetic("assistant", ite
 ---@return avante.HistoryMessage
 function M:new_user_synthetic(item) return M:new_synthetic("user", item) end
 
+---Updates content of a message as long as it is a simple text (or empty).
+---@param new_content string
+function M:update_content(new_content)
+  assert(type(self.message.content) == "string", "can only update content of simple string messages")
+  self.message.content = new_content
+end
+
 return M

lua/avante/llm.lua

@@ -979,7 +979,7 @@ function M._stream(opts)
               opts.on_chunk(prefix .. "\n" .. msg_content .. "\n")
             end
             if opts.on_messages_add and message then
-              message.message.content = "\n\n" .. msg_content
+              message:update_content("\n\n" .. msg_content)
               opts.on_messages_add({ message })
             end