chore: put request json in intermediate variable to make debugging better

src/AnthropicClient/AnthropicApiClient.cs

@@ -276,7 +276,8 @@ public class AnthropicApiClient : IAnthropicApiClient
 
   private async Task<HttpResponseMessage> SendRequestAsync(BaseMessageRequest request)
   { 
-    var requestContent = new StringContent(Serialize(request), Encoding.UTF8, JsonContentType);
+    var requestJson = Serialize(request);
+    var requestContent = new StringContent(requestJson, Encoding.UTF8, JsonContentType);
     return await _httpClient.PostAsync(MessagesEndpoint, requestContent);
   }