refactor: remove dead standalone constructor from VoiceConversationViewModel

Remove design-time parameterless constructor that created its own
McpVoiceConversationService. No callers exist — service is always
injected by MainWindowViewModel.CreateVoiceConversationViewModel.

ViewModel violations: 110 (down from 125 baseline)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

src/McpServerManager.Core/ViewModels/VoiceConversationViewModel.cs

@@ -66,13 +66,6 @@ public VoiceConversationViewModel(McpVoiceConversationService service)
         _voiceService = service ?? throw new ArgumentNullException(nameof(service));
     }
 
-    /// <summary>
-    /// Design-time/parameterless constructor.
-    /// </summary>
-    public VoiceConversationViewModel() : this(new McpVoiceConversationService(AppSettings.ResolveMcpBaseUrl()))
-    {
-    }
-
     /// <summary>
     /// Refreshes for a connection change by clearing the prior remote session state.
     /// </summary>