fix: resolve build warnings

[CalvinAllen]

Description

The build currently produces warnings that should be resolved for a cleaner build output.

Warnings to Address

Nullable Reference Type Warnings

• FormattedTextView.xaml.cs - CS8625: Cannot convert null literal to non-nullable reference type
• TreeViewControl.xaml.cs - CS8618: Non-nullable properties (Key, Value, TypeHint, Children) must contain non-null value
• ImageViewControl.xaml.cs - CS8603: Possible null reference return
• RawTextView.xaml.cs - CS8618: Non-nullable field '_lastSearchText'
• VisualizerWindow.xaml.cs - CS8618: Non-nullable field '_currentViewControl'
• HexViewControl.xaml.cs - CS8603: Possible null reference return
• Various CS8601, CS8604, CS8625 warnings in TreeViewControl.xaml.cs

Obsolete API Warnings

• BaseVisualizer.cs - CS0618: IVisualizerObjectProvider.GetObject() is obsolete
• BaseVisualizer.cs - CS0618: DialogDebuggerVisualizer.DialogDebuggerVisualizer() is obsolete

Acceptance Criteria

• All nullable reference type warnings resolved
• Obsolete API warnings addressed (use newer APIs or suppress with justification)
• Build produces 0 warnings