Function get_map_model 

pub fn get_map_model(main_model: &str) -> String

Model for Map stage in MapReduce summarization. SKILLLITE_MAP_MODEL. When set, Map (per-chunk summarization) uses this cheaper model; Reduce (merge) uses main model. E.g. qwen-plus, gemini-1.5-flash. If unset, both stages use main model.