Copying an enum or set type column into a string type column results in incorrect conversion

Discovered with #9594 (https://github.com/dolthub/dolt/issues/9594#issuecomment-3130323046)

Also happens with set types

dolt
```
tmp/main*> create table set_table(pk int primary key, c0 set('abc','def','ghi'));
Query OK, 0 rows affected (0.01 sec)
tmp/main*> insert into set_table values(0, 'abc,def'),(1,'def'),(2,'ghi');
Query OK, 3 rows affected (0.01 sec)
tmp/main*> create table string_table(pk int primary key, c0 varchar(20));
Query OK, 0 rows affected (0.01 sec)
tmp/main*> insert into string_table select * from set_table;
Query OK, 3 rows affected (0.01 sec)
tmp/main*> select * from string_table;
+----+----+
| pk | c0 |
+----+----+
| 0 | 3 |
| 1 | 2 |
| 2 | 4 |
+----+----+
3 rows in set (0.00 sec)
```

mysql
```
mysql> create table set_Table(pk int primary key, c0 set('abc','def','ghi'));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into set_table values (0, 'abc,def'),(1, 'def'),(2,'ghi');
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> create table string_table(pk int primary key, c0 varchar(20));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into string_table select * from set_table;
Query OK, 3 rows affected (0.00 sec)
Records: 3 Duplicates: 0 Warnings: 0

mysql> select * from string_table;
+----+---------+
| pk | c0 |
+----+---------+
| 0 | abc,def |
| 1 | def |
| 2 | ghi |
+----+---------+
3 rows in set (0.00 sec)
```

The column values are converted in `rowexec.insertIter.Next` when `col.Type.Convert` is called. We should be using `TypeAwareConversion` here but there's currently not an easy way to get the type of the source column.