Prepared statement uses stale div_precision_increment value after session variable change

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
PREPARE s FROM 'SELECT 1/3';
EXECUTE s;
SET SESSION div_precision_increment = 8;
EXECUTE s;
```

### 2. What did you expect to see? (Required)
After executing:
`SET SESSION div_precision_increment = 8;`
the prepared statement should use the updated session value when executed.
**Expected Result:** 0.33333333

### 3. What did you see instead (Required)
The prepared statement continues to use the previous value of div_precision_increment.
**Actual Result:** 0.3333

### 4. What is your TiDB version? (Required)
8.0.11-TiDB-v8.5.8

Start by running the reported PREPARE, EXECUTE, and SET SESSION div_precision_increment sequence against TiDB, then trace prepared-statement handling of session variables. Add a regression test for this sequence and verify that the second execution returns 0.33333333 rather than 0.3333.

Written by the indexing model from the issue text.