Update default model to gpt-4o-mini

and give gpt-4o as the common alternative.

src/chap/backends/openai_chatgpt.py

@@ -66,8 +66,8 @@ def from_model(cls, model: str) -> "EncodingMeta":
 class ChatGPT:
     @dataclass
     class Parameters:
-        model: str = "gpt-4-turbo"
-        """The model to use. The most common alternative value is 'gpt-3.5-turbo'."""
+        model: str = "gpt-4o-mini"
+        """The model to use. The most common alternative value is 'gpt-4o'."""
 
         max_request_tokens: int = 1024
         """The approximate greatest number of tokens to send in a request. When the session is long, the system prompt and 1 or more of the most recent interaction steps are sent."""