Made MudSelectExtended IsOpen flag public

[m-flex]

For discussion:

Made IsOpen property public in MudSelectExtended. This change allows external components or logic to determine whether the dropdown is currently open. It resolves the issue where OnOpen fires too late for a particular use-case (it fires after the content is rendered). By making IsOpen public, you can now directly check the dropdown's state and conditionally render components in the dropdown, ensuring it appears immediately when the dropdown opens.

[mckaragoz]

Understand. But we generally use methods for this, like

public bool GetOpenState()
{
    return _isOpen;
}

If you don't want to set the property, calling via method makes sense for me.

[m-flex]

That makes sense, I've amended the PR as per your suggestions.

[mckaragoz]

Thanks 🥇