refactor(sqlparser): rename parse_string_agg to `parse_string_conca…

…t` (risingwavelabs#3733)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

src/sqlparser/tests/sqlparser_common.rs

@@ -800,7 +800,7 @@ fn parse_in_subquery() {
 }
 
 #[test]
-fn parse_string_agg() {
+fn parse_string_concat() {
     let sql = "SELECT a || b";
 
     let select = verified_only_select(sql);