Cast json to datetime didn't have a correct fsp

[YangKeao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t (j json);
insert into t values (cast(cast("2024-10-24 11:11:11.12346" as datetime(6)) as json));
select cast(j as datetime(6)) from t;

2. What did you expect to see? (Required)

+----------------------------+
| cast(j as datetime(6))     |
+----------------------------+
| 2024-10-24 11:11:11.123460 |
+----------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

+------------------------+
| cast(j as datetime(6)) |
+------------------------+
| 2024-10-24 11:11:11    |
+------------------------+
1 row in set (0.00 sec)

Actually, there is another bug: insert into t values (cast("2024-10-24 11:11:11.12346" as datetime(6))); will fail on MySQL, but it successes on TiDB.

4. What is your TiDB version? (Required)

[YangKeao]

It doesn't affect the real data, so it doesn't affect the unqiueness or any other property of MVI. Not a big issue.

I found this issue when I was trying to fix #53243. I'll also fix this issue later.