Macro execute_function_calls 

execute_function_calls!() { /* proc-macro */ }

• Provide all functions to be called execute_function_calls!(session, f1, f2...)
• Returns Vec<Option<Result<serde_json::Value, String>>>
• Returned vec length always equals the number of functions passed
• None if f_i was not called by Gemini if function don’t return type Result, it always return Result::Ok(value)
• Session struct is automatically updated with FunctionResponse only for Ok result