inconsistency update when primary key contains timestamp of DST

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
> create table t(id timestamp primary key, v int);
> set @@time_zone='UTC';
> insert into t values('2025-11-02 09:30:00', 1);
> set @@time_zone='America/Los_Angeles';
> update t set v=v+1;
```

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

Update should success and it should not produce new record.

### 3. What did you see instead (Required)

assertion failed when updating:
```sql
TiDB root@127.0.0.1:test> update t set v=v+1;
(8141, 'assertion failed: key: 74800000000000032c5f720419b8048780000000, assertion: Exist, start_ts: 457607176025014273, existing start ts: 0, existing commit ts: 0')
```

If we turn off assertion check, update can success but it produces a new record:

```sql
TiDB root@127.0.0.1:test> select * from t;
+---------------------+---+
| id | v |
+---------------------+---+
| 2025-11-02 01:30:00 | 1 |
+---------------------+---+
1 row in set
Time: 0.004s
TiDB root@127.0.0.1:test> set @@tidb_txn_assertion_level='OFF';
Query OK, 0 rows affected
Time: 0.001s
TiDB root@127.0.0.1:test> update t set v=v+1;
Query OK, 1 row affected
Time: 0.003s
TiDB root@127.0.0.1:test> select * from t;
+---------------------+---+
| id | v |
+---------------------+---+
| 2025-11-02 01:30:00 | 2 |
| 2025-11-02 01:30:00 | 1 |
+---------------------+---+
2 rows in set
Time: 0.003s
```

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

found in master, but it may affects all versions.