Add system msg when calling inside the assistant tool loop (#4308)

[gziz]

Why are these changes needed?

The AssistantAgent determines when it needs to use tools. After all necessary tools are used during the tool execution while loop the agent makes one model_client call to generate a response based on the tool results. The issue is that this model client call doesn't include the system message.

Related issue number

4308

Checks

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• [ ] I've made sure all auto checks have passed.