Fix DeleteTaskPushNotificationConfigAsync throwing on valid null result

DeleteTaskPushNotificationConfig is the only A2A method with a void
result. The generic JSON-RPC send helper unconditionally deserialized
the response result and threw A2AException when it was null, so the
client rejected the spec-compliant null-result success response that
A2AJsonRpcProcessor itself produces.

Split the transport/error handling into SendJsonRpcRequestCoreAsync and
route delete through a void-result overload that skips result
deserialization. Fixes #429.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: MertBasar0

src/A2A/Client/A2AClient.cs

@@ -91,7 +91,7 @@ public async Task<ListTaskPushNotificationConfigResponse> ListTaskPushNotificati
     /// <inheritdoc />
     public async Task DeleteTaskPushNotificationConfigAsync(DeleteTaskPushNotificationConfigRequest request, CancellationToken cancellationToken = default)
     {
-        await SendJsonRpcRequestAsync<object>(A2AMethods.DeleteTaskPushNotificationConfig, request, cancellationToken).ConfigureAwait(false);
+        await SendJsonRpcRequestAsync(A2AMethods.DeleteTaskPushNotificationConfig, request, cancellationToken).ConfigureAwait(false);
     }
 
     /// <inheritdoc />
@@ -111,6 +111,25 @@ public void Dispose()
     [UnconditionalSuppressMessage("AOT", "IL2026:RequiresUnreferencedCode", Justification = "All types are registered in source-generated JsonContext.")]
     [UnconditionalSuppressMessage("AOT", "IL3050:RequiresDynamicCode", Justification = "All types are registered in source-generated JsonContext.")]
     private async Task<TResult> SendJsonRpcRequestAsync<TResult>(string method, object? @params, CancellationToken cancellationToken)
+    {
+        var rpcResponse = await SendJsonRpcRequestCoreAsync(method, @params, cancellationToken).ConfigureAwait(false);
+
+        return rpcResponse.Result.Deserialize<TResult>(A2AJsonUtilities.DefaultOptions)
+            ?? throw new A2AException("Failed to deserialize JSON-RPC result.", A2AErrorCode.InternalError);
+    }
+
+    /// <summary>Sends a JSON-RPC request for methods whose successful response carries no result, e.g. a null <c>result</c> member.</summary>
+    /// <param name="method">The JSON-RPC method name.</param>
+    /// <param name="params">The request parameters to serialize, if any.</param>
+    /// <param name="cancellationToken">A token to cancel the operation.</param>
+    private async Task SendJsonRpcRequestAsync(string method, object? @params, CancellationToken cancellationToken)
+    {
+        _ = await SendJsonRpcRequestCoreAsync(method, @params, cancellationToken).ConfigureAwait(false);
+    }
+
+    [UnconditionalSuppressMessage("AOT", "IL2026:RequiresUnreferencedCode", Justification = "All types are registered in source-generated JsonContext.")]
+    [UnconditionalSuppressMessage("AOT", "IL3050:RequiresDynamicCode", Justification = "All types are registered in source-generated JsonContext.")]
+    private async Task<JsonRpcResponse> SendJsonRpcRequestCoreAsync(string method, object? @params, CancellationToken cancellationToken)
     {
         using var activity = A2ADiagnostics.Source.StartActivity($"A2AClient/{method}", ActivityKind.Client);
         var stopwatch = Stopwatch.StartNew();
@@ -146,8 +165,7 @@ private async Task<TResult> SendJsonRpcRequestAsync<TResult>(string method, obje
                 throw new A2AException(error.Message, (A2AErrorCode)error.Code);
             }
 
-            return rpcResponse.Result.Deserialize<TResult>(A2AJsonUtilities.DefaultOptions)
-                ?? throw new A2AException("Failed to deserialize JSON-RPC result.", A2AErrorCode.InternalError);
+            return rpcResponse;
         }
         catch (Exception ex)
         {

tests/A2A.UnitTests/Client/A2AClientTests.cs

@@ -372,6 +372,16 @@ public async Task DeletePushNotificationConfigAsync_SendsCorrectMethod()
         Assert.Equal(A2AMethods.DeleteTaskPushNotificationConfig, requestJson.RootElement.GetProperty("method").GetString());
     }
 
+    [Fact]
+    public async Task DeletePushNotificationConfigAsync_CompletesOnNullResult()
+    {
+        // A2AJsonRpcProcessor answers delete with a success response whose result is null,
+        // which is valid per JSON-RPC 2.0 for the only void-result A2A method.
+        var sut = CreateA2AClient(new JsonRpcResponse { Id = "test-id", Result = null });
+
+        await sut.DeleteTaskPushNotificationConfigAsync(new DeleteTaskPushNotificationConfigRequest { Id = "cfg-1", TaskId = "t-1" });
+    }
+
     private static A2AClient CreateA2AClient(object result, Action<HttpRequestMessage>? onRequest = null, bool isSse = false)
     {
         var response = new JsonRpcResponse