Fix NullReferenceException in ProviderBrowser.LevelSelected when deselecting level

[Copilot]

The LevelSelected event handler in ProviderBrowser.xaml.cs was calling LevelListBox.SelectedItem.ToString() without null checking, causing a crash when users attempted to deselect the selected level by holding Ctrl and clicking it.

Problem

When a user holds Ctrl and clicks the selected item in the Level ListBox:

1. The ListBox.SelectedItem becomes null
2. The LevelSelected event handler tries to call SelectedItem.ToString()
3. This causes a NullReferenceException

Solution

Added defensive null checking that prevents deselection and maintains the requirement that exactly one level must always be selected:

private void LevelSelected(object sender, SelectionChangedEventArgs e)
{
    // Ensure at least one level is always selected
    if (LevelListBox.SelectedItem == null)
    {
        // If nothing is selected, reselect the previous level or default to "Verbose"
        string levelToSelect = !string.IsNullOrEmpty(m_level) ? m_level : "Verbose";
        LevelListBox.SelectedItem = levelToSelect;
        return;
    }
    
    m_level = LevelListBox.SelectedItem.ToString();
    updateDisplays();
}

This approach:

• Prevents the NullReferenceException
• Maintains the invariant that exactly one level is always selected
• Reselects the user's previous choice when they attempt to deselect
• Aligns with the maintainer's guidance that "you can't unselect the selected option"

Fixes #983.

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[brianrob]

Copilot Post-Mortem:

• Great example of a simple fix that didn't require me to debug to find the code and implement a fix.
• Simple to test and validate since it's a small, scoped change.

[marklio]

This looks like a reasonable fall-back should the state get unselected, but it would seem to indicate the wrong UI paradigm is being used for selection. Consider a follow-up change to the UI to make the selection more natural and UI-enforced (single-selected radio-button? drop-down? spinner? etc.)

[brianrob]

Yeah, it is kind of funny that it's a multi-select control but only allows one selection. Will consider a follow-up change. Thanks for the review!