[Bug]: GREATEST/LEAST reject mixed string-date arguments unlike MySQL

## Bug Report

### MatrixOne Version

Latest `origin/main` verified locally at `200bfa7e0efb`.

### Reproduce Steps

```sql
set session sql_mode = '';

select greatest(cast('2024-01-02' as date), cast('2023-12-31' as date)) as greatest_date,
least(cast('2024-01-02' as date), cast('2023-12-31' as date)) as least_date;

select greatest('2024-01-02', cast('2024-01-10' as date)) as greatest_str_date,
least('2024-01-02', cast('2024-01-10' as date)) as least_str_date;
```

### Expected Behavior

MySQL supports both same-type temporal arguments and mixed string/date arguments:

```text
greatest_date least_date
2024-01-02 2023-12-31

greatest_str_date least_str_date
2024-01-10 2024-01-02
```

### Actual Behavior

MatrixOne supports the same-type temporal row but rejects the mixed string/date row:

```text
greatest_date least_date
2024-01-02 2023-12-31

ERROR 20203 (HY000): invalid argument function greatest, bad value [VARCHAR DATE]
```

### Notes

Found during MySQL compatibility Module 7 exploration. Existing #25215 covers mixed string-number arguments for `GREATEST/LEAST`; this issue tracks the separate mixed string/date path.