.Net Handlebars planner shouldn't utilize system messages

[Krzysztof318]

Current Handlebars planner prompt uses many messages with the "system" role. It causes that planner is unusable with other models than openai models. Many models like for example gemini support only user and assistant message role.

We should change prompt to utilize only messages with user and assistant roles.

[matthewbolanos]

Closing as a duplicate of #4870. I'll respond in the other issue, but our plan is to get additional models that have Chat Completion support so that you can use the Handlebars planner with other models other than OpenAI.