Not well handle \ in REGEXP function

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

In a normal query, `\'` are used to represent `'`, however, this will trigger syntax error in a prepared statement

```
SELECT '\''; -- '
PREPARE prepare_query FROM "SELECT '\''"; -- You have an error in your SQL syntax; check the manual that corresponds to your TiDB version for the right syntax to use line 1 column 10 near "'''"
```

### 2. What did you expect to see? (Required)
No error.

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

### 4. What is your TiDB version? (Required)
Release Version: v9.0.0-beta.2.pre-196-g312226f869\nEdition: Community\nGit Commit Hash: 312226f869d56922135e1c177cac6c90319c3134\nGit Branch: HEAD\nUTC Build Time: 2025-09-21 08:27:14\nGoVersion: go1.23.11\nRace Enabled: false\nCheck Table Before Drop: false\nStore: unistore\nKernel Type: Classic