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

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[yeandy]

LGTM

[rysweet]

Thanks @gziz