bug: per prompt target is ignored and system prompt is getting used instead

[adilhafeez]

Per system prompt is getting ignored and system prompt is used for all prompts.

[adilhafeez]

Fix is in #368 but testing is manual until mock out upstream llm. For now here is how I have verified this code change,

1. start currency exchange demo
2. start model server in foreground

   $ archgw up --service model_server --foreground
   

3. start archgw from currency exchange in foreground

   $ archgw up --service archgw --foreground
   

4. send following rest request to arch gateway,

   $ curl --header 'Content-Type: application/json' \
     --data '{"messages": [{"role": "user","content": "can you please convert 100 jpy"}]}' \
     http://localhost:10000/v1/chat/completions"
   

5. after the request complete head over to archgw console logs and look for line archgw => llm request. There you should see system prompt with message You are a helpful assistant. Show me the currency symbol you want to convert from USD.\n