refactor(history): change HistoryMessage:new() to accept role and item

Change the constructor to accept role and a single content item instead
of raw AvanteLLMMessage instance. This will help when we will start
changing message.content from being a string or a list of tables to just
a string or a single table.

lua/avante/suggestion.lua

@@ -231,7 +231,10 @@ L5:     pass
     },
   }
 
-  local history_messages = vim.iter(llm_messages):map(function(msg) return HistoryMessage:new(msg) end):totable()
+  local history_messages = vim
+    .iter(llm_messages)
+    :map(function(msg) return HistoryMessage:new(msg.role, msg.content) end)
+    :totable()
 
   local diagnostics = Utils.lsp.get_diagnostics(bufnr)