C#: Fix SearchResult.Found using dynamic dispatch instead of reflection

SearchResult.Found used GetMethod reflection to call WithMarkers on
concrete tree types. This silently returned the same instance when
GetMethod failed, breaking PreconditionsCheckTest assertions that
were previously masked by Gradle build cache.

Replace the fragile reflection with dynamic dispatch, which reliably
resolves WithMarkers on any concrete type at runtime.

Author: steve-aom-elliott

rewrite-csharp/csharp/OpenRewrite/Core/SearchResult.cs

@@ -33,8 +33,7 @@ public sealed class SearchResult(Guid id, string? description) : Marker, IRpcCod
     public static T Found<T>(T tree, string? description = null) where T : J
     {
         var newMarkers = tree.Markers.Add(new SearchResult(Guid.NewGuid(), description));
-        var withMarkers = tree.GetType().GetMethod("WithMarkers", [typeof(Markers)]);
-        return withMarkers != null ? (T)withMarkers.Invoke(tree, [newMarkers])! : tree;
+        return (T)(object)((dynamic)tree).WithMarkers(newMarkers);
     }
 
     public void RpcSend(SearchResult after, RpcSendQueue q)