Change default OpenAI model (gpt-4.1 -> gpt-5)

Ref: https://openai.com/index/introducing-gpt-5/

docs/sample-provider-configs/openai.yml

@@ -1,7 +1,7 @@
 base_url: https://api.openai.com/v1
 api_key: YOUR_API_KEY_HERE
 text_generation:
-  model_id: gpt-4.1
+  model_id: gpt-5
   prompt: "You are a brief, but helpful bot called {{ baibot_name }} powered by the {{ baibot_model_id }} model. The date/time of this conversation's start is: {{ baibot_conversation_start_time_utc }}."
   temperature: 1.0
   max_response_tokens: 16384