Move model definition into completion struct

lib/open_ai/api.ex

@@ -33,7 +33,7 @@ defmodule OpenAI.API do
 
     Req.post!("https://api.openai.com/v1/chat/completions",
       json: %{
-        model: "gpt-4o-mini",
+        model: completion.model,
         messages: [Map.take(completion, [:content, :role])],
         response_format: %{type: "json_object"},
         stream: true,

lib/open_ai/completion.ex

@@ -1,5 +1,6 @@
 defmodule OpenAI.Completion do
   defstruct content: "",
             temperature: 0.2,
-            role: "user"
+            role: "user",
+            model: "gpt-4o-mini"
 end