result of LENGTH(CAST(1e100 AS CHAR)) is incosistent with mysql

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
SELECT LENGTH(CAST(1e100 AS CHAR));
```
### 2. What did you expect to see? (Required)
```
MySQL [(none)]> SELECT LENGTH(CAST(1e100 AS CHAR));
+-----------------------------+
| LENGTH(CAST(1e100 AS CHAR)) |
+-----------------------------+
| 5 |
+-----------------------------+
1 row in set (0.00 sec)
```

### 3. What did you see instead (Required)
```
MySQL [testdb_1747239281507]> SELECT LENGTH(CAST(1e100 AS CHAR));
+-----------------------------+
| LENGTH(CAST(1e100 AS CHAR)) |
+-----------------------------+
| 101 |
+-----------------------------+
1 row in set (0.01 sec)
```

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

```
MySQL [testdb_1747239281507]> select tidb_version();
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version() |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: v8.5.1
Edition: Community
Git Commit Hash: fea86c8e35ad4a86a5e1160701f99493c2ee547c
Git Branch: HEAD
UTC Build Time: 2025-01-16 07:38:34
GoVersion: go1.23.4
Race Enabled: false
Check Table Before Drop: false
Store: tikv |
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
```

Start by running SELECT LENGTH(CAST(1e100 AS CHAR)); on TiDB and MySQL, then trace the CAST and LENGTH handling in the Go codebase. Done means TiDB returns the MySQL-compatible result of 5 and includes regression coverage for this query.

Written by the indexing model from the issue text.