Fix: handle JsonSerializerOptions without TypeInfoResolver

Added a helper method which ensures that custom options are properly configured with required TypeInfoResolvers.

Fixes #1150

Author: GeorgiPopovIT

src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

@@ -580,4 +580,22 @@ private static CallToolResult ConvertAIContentEnumerableToCallToolResult(IEnumer
             IsError = allErrorContent && hasAny
         };
     }
+
+    private static JsonSerializerOptions GetSerializerOptions(JsonSerializerOptions? customOptions)
+    {
+        if (customOptions is null)
+        {
+            return McpJsonUtilities.DefaultOptions;
+        }
+
+        if (customOptions.TypeInfoResolver is not null)
+        {
+            return customOptions;
+        }
+
+        customOptions.TypeInfoResolverChain.Add(McpJsonUtilities.JsonContext.Default);
+        customOptions.TypeInfoResolverChain.Add(AIJsonUtilities.DefaultOptions.TypeInfoResolver!);
+
+        return customOptions;
+    }
 }