Send a message to Google Gemini and get a response.

Args:
    params (GeminiChatInput): Chat parameters including:
        - prompt (str): The prompt to send
        - file (Optional[list[str]]): Files to include (text, code, images)
        - session_id (Optional[str]): Session ID for multi-turn chat, use 'last' for recent
        - model (Optional[str]): Override model selection
        - system_prompt (Optional[str]): System context
        - temperature (Optional[float]): Creativity (0.0-2.0)
        - max_tokens (Optional[int]): Max response length
        - response_format: Output format - 'markdown' or 'json'

Returns:
    str: Response with SESSION_ID for continuation.

Examples:
    - Simple: prompt="What is AI?"
    - With file: prompt="Review", file=["main.py"]
    - With image: prompt="Describe", file=["photo.jpg"]
    - Continue: prompt="Tell me more", session_id="last"