tests: cover null responses

src/AnthropicClient/AnthropicApiClient.cs

@@ -1,7 +1,6 @@
 using System.Net.Http.Headers;
 using System.Text;
 using System.Text.Json;
-using System.Threading.Tasks;
 
 using AnthropicClient.Json;
 using AnthropicClient.Models;

src/AnthropicClient/Json/ContentDeltaConverter.cs

@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 

src/AnthropicClient/Json/EventDataConverter.cs

@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using System.Text.Json;
 using System.Text.Json.Serialization;