[Bug]: CHAR_LENGTH over binary string counts characters instead of bytes

### Environment

- Branch: latest `main`
- Commit tested locally: `200bfa7e0efb` (`fix: improve skills SKILL.md compatibility format (#25271)`)
- MySQL oracle: local MySQL 8.x via `mysql -u root -p...`
- MO: `8.0.30-MatrixOne-v1.3.0`

### Steps to Reproduce

```sql
select length(_binary '你好') as byte_len,
char_length(_binary '你好') as char_len;
```

### MySQL Result

```text
byte_len char_len
6 6
```

For a binary string, MySQL counts bytes for `CHAR_LENGTH()`.

### MatrixOne Result

```text
byte_len char_len
6 2
```

MatrixOne appears to count Unicode characters even though the literal is explicitly `_binary`.

### Expected Behavior

For binary strings, `CHAR_LENGTH()` / `CHARACTER_LENGTH()` should match MySQL and count bytes, not decoded characters.

### Notes

Found during active MySQL compatibility exploration, Module 4: String / Charset / Collation. I searched existing issues for `CHAR_LENGTH binary` and did not find a duplicate.