fix: Ensure headers are lists in CalendarBase.cs

Converted results of GetColumnHeaders() and GetRowHeaders() to lists before assigning to _columnHeaders and _rowHeaders in CalendarBase.cs. This change ensures consistency and allows for list-specific operations.

src/MyNet.Wpf/Controls/CalendarBase.cs

@@ -1222,8 +1222,8 @@ protected virtual void BuildCore(CancellationToken cancellationToken)
 
             cancellationToken.ThrowIfCancellationRequested();
 
-            _columnHeaders.Set(GetColumnHeaders());
-            _rowHeaders.Set(GetRowHeaders());
+            _columnHeaders.Set(GetColumnHeaders().ToList());
+            _rowHeaders.Set(GetRowHeaders().ToList());
 
             if (_displayDates.Count != dates.Count)
             {