Fix missing resultType on complete result responses (#1684)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Tarek Mahmoud Sayed <tarekms@ntdev.microsoft.com>

tests/ModelContextProtocol.Tests/Server/McpServerTaskTests.cs

@@ -58,6 +58,21 @@ public class McpServerTaskTests : ClientServerTestBase
                     };
                 }
 
+                if (toolName == "async-tool-default-resulttype")
+                {
+                    // Intentionally leaves ResultType unset so the server boundary is responsible for
+                    // filling in "task" on the wire.
+                    var taskId = store.CreateTask();
+                    return new CreateTaskResult
+                    {
+                        TaskId = taskId,
+                        Status = McpTaskStatus.Working,
+                        CreatedAt = DateTimeOffset.UtcNow,
+                        LastUpdatedAt = DateTimeOffset.UtcNow,
+                        PollIntervalMs = 50,
+                    };
+                }
+
                 if (toolName == "input-required-tool")
                 {
                     var taskId = store.CreateTask(McpTaskStatus.InputRequired);
@@ -292,6 +307,19 @@ public class McpServerTaskTests : ClientServerTestBase
         Assert.Equal("task", augmented.TaskCreated!.ResultType);
     }
 
+    [Fact]
+    public async Task CreateTaskResult_WithoutExplicitResultType_ServerFillsTask()
+    {
+        await using var client = await CreateMcpClientForServer();
+
+        var augmented = await client.CallToolRawAsync(
+            new CallToolRequestParams { Name = "async-tool-default-resulttype" },
+            TestContext.Current.CancellationToken);
+
+        Assert.True(augmented.IsTask);
+        Assert.Equal("task", augmented.TaskCreated!.ResultType);
+    }
+
     [Fact]
     public async Task GetTaskAsync_ImmediatelyAfterCreate_Resolves()
     {