The `timestampadd()` function handles time precision abnormally

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
create table lrr_test(`COL1` datetime(2) NOT NULL);
insert into lrr_test values('2024-01-01 00:00:00.32');
insert into lrr_test values('2024-01-01 00:02:00.00');
select col1, timestampadd(hour, 1, col1) from lrr_test;
```

### 2. What did you expect to see? (Required)
Mysql：
+------------------------+-----------------------------+
| col1 | timestampadd(hour, 1, col1) |
+------------------------+-----------------------------+
| 2024-01-01 00:02:00.00 | 2024-01-01 01:02:00.00 |
| 2024-01-01 00:00:00.32 | 2024-01-01 01:00:00.32 |
+------------------------+-----------------------------+
After timestampadd() function processing, two decimal places are retained

### 3. What did you see instead (Required)
+------------------------+-----------------------------+
| col1 | timestampadd(hour, 1, col1) |
+------------------------+-----------------------------+
| 2024-01-01 00:00:00.32 | 2024-01-01 01:00:00.320000 |
| 2024-01-01 00:02:00.00 | 2024-01-01 01:02:00 |
+------------------------+-----------------------------+
Some retain six decimal places. If it is `.00`, no decimal places are retained.

### 4. What is your TiDB version? (Required)

Release Version: v8.5.0-alpha-7-g22a175b384
Edition: Community
Git Commit Hash: 22a175b384d6e171641e0aa0d3d8b9c15a92a935
Git Branch: HEAD
UTC Build Time: 2024-10-30 05:21:13
GoVersion: go1.23.2
Race Enabled: false
Check Table Before Drop: false
Store: tikv