Bug: Database dateFormat['datetime'] does not support millisecond or microsecond values

### PHP Version

8.4

### CodeIgniter4 Version

4.7.4

### CodeIgniter4 Installation Method

Composer (as dependency to an existing project)

### Which operating systems have you tested for this bug?

Windows

### Which server did you use?

cli-server (PHP built-in webserver)

### Environment

development

### Database

MySQL 8.4.9

### What happened?

When setting the database connection in app\Config\Database.php, I am using the following dateFormat definition to get millisecond precision in my dates:
```
'dateFormat' => [
'date' => 'Y-m-d',
'datetime' => 'Y-m-d H:i:s.v',
'time' => 'H:i:s',
],
```
My model then configures the following for timestamps:
```
protected $useTimestamps = true;
protected $dateFormat = 'datetime';
protected $createdField = 'created_at';
```
After inserting an entry the table, the `created_at` column does not include the milliseconds. For example, the column has the value **2026-09-11 10:17:35** however because the column definition in MySQL is timestamp(6) the column should have the value **2026-09-11 10:17:35.509000**.

### Steps to Reproduce

Set the datetime mask as above and the model variables as above and insert a value into your table.

### Expected Output

I expected the `created_at` column to include the milliseconds.

### Anything else?

I have resolved this problem by overriding the `setDate()` function in the BaseModel as follows.

My model class is defined as:
```
class TransactionHistoryModel extends \App\Models\BaseModel {
...
}
```
I have then created my own BaseModel with the setDate override defined as:
```
namespace App\Models;

class BaseModel extends Model {

/**
* Sets the timestamp or current timestamp if null value is passed.
*
* @param int|null $userDate An optional PHP timestamp to be converted
*
* @return int|string
*
* @throws ModelException
*/
protected function setDate(?int $userDate = null)
{
$currentDate = $userDate ? Time::createFromTimestamp($userDate) : Time::now();

return $this->timeToDate($currentDate);
}
}
```
This solution works.

Start with the dateFormat configuration in app\Config\Database.php and trace the model's setDate() and timeToDate() entry points. Reproduce the timestamped insert on MySQL using a timestamp(6) column and the reported datetime mask. Done means created_at preserves the configured millisecond or microsecond precision.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.