sqlparser: reject unqualified * after comma in SELECT list

MySQL only allows unqualified `*` as the first expression in a SELECT
list. After a comma, only qualified `table.*` is permitted. Add a check
in the `select_expression_list` grammar rule to reject unqualified `*`
when it appears after a comma, matching MySQL behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Arthur Schreiber <arthur@planetscale.com>

Author: arthurschreiber

go/vt/sqlparser/parse_test.go

@@ -6633,6 +6633,12 @@ var invalidSQL = []struct {
 }, {
 	input:  "insert into t1 (a1) values row('a'), ('b')",
 	output: "syntax error at position 39",
+}, {
+	input:  "select a, * from t",
+	output: "syntax error: unexpected '*' at position 12",
+}, {
+	input:  "select *, * from t",
+	output: "syntax error: unexpected '*' at position 12",
 }}
 
 func TestErrors(t *testing.T) {