Function execute_aggregate 

pub fn execute_aggregate(
    select: &SelectStmt,
    rows: &[HashMap<String, SochValue>],
    params: &[SochValue],
    limit: Option<usize>,
    offset: Option<usize>,
) -> SqlResult<ExecutionResult>

Execute aggregation over materialized input rows (already WHERE-filtered).

Handles GROUP BY, all aggregate accumulation, HAVING, ORDER BY, OFFSET/LIMIT, and final projection. Returns ExecutionResult::Rows.