Fix(mysql)!: preserve TIMESTAMP on roundtrip

[georgesittas]

Fixes #5127

Today, we convert TIMESTAMP into DATETIME for MySQL. This is incorrect, and I'll use the following example to explain why:

mysql> CREATE TABLE foo (ts TIMESTAMP, dt DATETIME);
Query OK, 0 rows affected (0.028 sec)

mysql> INSERT INTO foo (ts, dt) VALUES ('2025-05-30 12:00:00', '2025-05-30 12:00:00');
Query OK, 1 row affected (0.005 sec)

mysql> SELECT * FROM foo WHERE ts = '2025-05-30 12:00:00';
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2025-05-30 12:00:00 | 2025-05-30 12:00:00 |
+---------------------+---------------------+
1 row in set (0.001 sec)

mysql> SELECT * FROM foo WHERE dt = '2025-05-30 12:00:00';
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2025-05-30 12:00:00 | 2025-05-30 12:00:00 |
+---------------------+---------------------+
1 row in set (0.001 sec)

mysql> SET time_zone = '-05:00'; -- TIMESTAMP values are time-zone aware, so this impacts them
Query OK, 0 rows affected (0.001 sec)

mysql> SELECT * FROM foo WHERE ts = '2025-05-30 12:00:00'; -- Notice how this doesn't return anything now
Empty set (0.001 sec)

mysql> SELECT * FROM foo WHERE dt = '2025-05-30 12:00:00'; -- Notice how DATETIME values are unaffected
+---------------------+---------------------+
| ts                  | dt                  |
+---------------------+---------------------+
| 2025-05-30 04:00:00 | 2025-05-30 12:00:00 |
+---------------------+---------------------+
1 row in set (0.000 sec)

By transpiling TIMESTAMP into DATETIME, SQLGlot incorrectly changes the type semantics. Specifically, with this transformation, the 2nd-to-last query would return 1 row if we used SQLGlot's transformed DDL, because we'd essentially be running the last query (ts and dt would be both DATETIME values).

Reference: https://dev.mysql.com/doc/refman/8.4/en/datetime.html