fix: make copy a dml statement for qualify_tables

Author: tobymao

sqlglot/expressions.py

@@ -2015,7 +2015,7 @@ class CopyParameter(Expression):
     arg_types = {"this": True, "expression": False, "expressions": False}
 
 
-class Copy(Expression):
+class Copy(DML):
     arg_types = {
         "this": True,
         "kind": True,

tests/fixtures/optimizer/qualify_tables.sql

@@ -177,3 +177,6 @@ SELECT a.id, a_2.id FROM c.db1.a AS a JOIN c.db2.a AS a_2 ON a.id = a_2.id;
 
 SELECT cat.db1.a.id, db2.a.id FROM cat.db1.a JOIN db2.a ON cat.db1.a.id = db2.a.id;
 SELECT a.id, a_2.id FROM cat.db1.a AS a JOIN c.db2.a AS a_2 ON a.id = a_2.id;
+
+COPY INTO (SELECT * FROM x) TO 'data' WITH (FORMAT 'CSV');
+COPY INTO (SELECT * FROM c.db.x AS x) TO 'data' WITH (FORMAT 'CSV');