Fix Gemini bug when system role is provided

[winsonluk]

Gemini doesn't support role=system.

When "SMART_LLM": "google_vertexai:gemini-1.5-flash-002" for example, the current report generation fails with "Error in generate_report".

This fix adds a fallback. It first attempts the original chat completion:

            messages=[                                                                              
                {"role": "system", "content": f"{agent_role_prompt}"},                              
                {"role": "user", "content": content},                                               
            ],

Upon failure, it retries with this new chat completion:

                messages=[                                                                          
                    {"role": "user", "content": f"{agent_role_prompt}\n\n{content}"},               
                ],

This results in a successful report generation when "SMART_LLM": "google_vertexai:gemini-1.5-flash-002".

[winsonluk]

@assafelovic short fix for when SMART_LLM fails with Gemini, thanks

[assafelovic]

Thanks for this!