Function is_aggregate_function 

pub fn is_aggregate_function(name: &str) -> bool

Checks if a function is an aggregate function (e.g., SUM, COUNT, AVG).

Aggregate functions combine multiple input rows into a single output value. This classification is used to detect aggregation in SELECT expressions and validate GROUP BY semantics.

The check is case-insensitive. Uses ASCII lowercase for performance since SQL function names are always ASCII.