Function builtin_map_reduce 

pub async fn builtin_map_reduce(
    args: &[DslValue],
    ctx: &ReasoningBuiltinContext,
) -> Result<DslValue>

Execute the map_reduce builtin: parallel fan-out + aggregate.

Arguments (named via map):

• inputs: list — items to process
• mapper: string — system prompt for the mapper
• reducer: string — system prompt for the reducer

Returns a map with keys: result, mapped_results.