chore: remove unused dead fields from VoiceConversationViewModel

Remove _mcpBaseUrl and _mcpApiKey fields that were assigned but never read,
eliminating CS0414 compiler warnings. These fields are vestiges of an earlier
refactoring where the service was configured with URL/key directly; now the
service is constructed externally and injected.

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

Author: sharpninja

src/McpServerManager.Core/ViewModels/VoiceConversationViewModel.cs

@@ -23,8 +23,6 @@ public partial class VoiceConversationViewModel : ViewModelBase
 
     private McpVoiceConversationService _voiceService;
     private CancellationTokenSource? _activeTurnCts;
-    private string _mcpBaseUrl;
-    private string? _mcpApiKey;
 
     [ObservableProperty] private string _sessionId = string.Empty;
     [ObservableProperty] private string _language = "en-US";
@@ -65,8 +63,6 @@ public string WorkspacePath
     /// </summary>
     public VoiceConversationViewModel(McpVoiceConversationService service)
     {
-        _mcpBaseUrl = "";
-        _mcpApiKey = null;
         _voiceService = service ?? throw new ArgumentNullException(nameof(service));
     }