Implement SEP-2663 Tasks Extension

Directory.Packages.props

@@ -25,6 +25,7 @@
   <!-- Product dependencies .NET Standard -->
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
     <PackageVersion Include="Microsoft.Bcl.Memory" Version="$(System10Version)" />
+    <PackageVersion Include="System.Collections.Immutable" Version="$(System10Version)" />
     <PackageVersion Include="System.Diagnostics.DiagnosticSource" Version="$(System10Version)" />
     <PackageVersion Include="System.IO.Pipelines" Version="$(System10Version)" />
     <PackageVersion Include="System.Text.Json" Version="$(System10Version)" />

ModelContextProtocol.slnx

@@ -45,11 +45,11 @@
     <Project Path="samples/ChatWithTools/ChatWithTools.csproj" />
     <Project Path="samples/EverythingServer/EverythingServer.csproj" />
     <Project Path="samples/InMemoryTransport/InMemoryTransport.csproj" />
-    <Project Path="samples/LongRunningTasks/LongRunningTasks.csproj" />
     <Project Path="samples/ProtectedMcpClient/ProtectedMcpClient.csproj" />
     <Project Path="samples/ProtectedMcpServer/ProtectedMcpServer.csproj" />
     <Project Path="samples/QuickstartClient/QuickstartClient.csproj" />
     <Project Path="samples/QuickstartWeatherServer/QuickstartWeatherServer.csproj" />
+    <Project Path="samples/TasksExtension/TasksExtension.csproj" />
     <Project Path="samples/TestServerWithHosting/TestServerWithHosting.csproj" />
   </Folder>
   <Folder Name="/Solution Items/">

docs/concepts/stateless/stateless.md

@@ -568,7 +568,7 @@ In stateless mode, each HTTP request creates and disposes a short-lived `McpServ
 
 ## Tasks and session modes
 
-[Tasks](xref:tasks) enable a "call-now, fetch-later" pattern for long-running tool calls. Task support depends on having an <xref:ModelContextProtocol.IMcpTaskStore> configured (`McpServerOptions.TaskStore`), and behavior differs between session modes.
+[Tasks](xref:tasks) enable a "call-now, fetch-later" pattern for long-running tool calls. Task support depends on having an <xref:ModelContextProtocol.Server.IMcpTaskStore> configured (`McpServerOptions.TaskStore`), and behavior differs between session modes.
 
 ### Stateless mode
 
@@ -578,9 +578,9 @@ In stateless mode, there is no `SessionId`, so the task store does not apply ses
 
 ### Stateful mode
 
-In stateful mode, the `IMcpTaskStore` receives the session's `SessionId` on every operation — `CreateTaskAsync`, `GetTaskAsync`, `ListTasksAsync`, `CancelTaskAsync`, etc. The built-in <xref:ModelContextProtocol.InMemoryMcpTaskStore> enforces session isolation: tasks created in one session cannot be accessed from another.
+In stateful mode, the `IMcpTaskStore` receives the session's `SessionId` on every operation — `CreateTaskAsync`, `GetTaskAsync`, `ListTasksAsync`, `CancelTaskAsync`, etc. The built-in <xref:ModelContextProtocol.Server.InMemoryMcpTaskStore> enforces session isolation: tasks created in one session cannot be accessed from another.
 
-Tasks can outlive individual HTTP requests because the tool executes in the background after returning the initial `CreateTaskResult`. Task cleanup is governed by the task's TTL (time-to-live), not by session termination. However, the `InMemoryMcpTaskStore` loses all tasks if the server process restarts. For durable tasks, implement a custom <xref:ModelContextProtocol.IMcpTaskStore> backed by an external store. See [Fault-tolerant task implementations](xref:tasks#fault-tolerant-task-implementations) for guidance.
+Tasks can outlive individual HTTP requests because the tool executes in the background after returning the initial `CreateTaskResult`. Task cleanup is governed by the task's TTL (time-to-live), not by session termination. However, the `InMemoryMcpTaskStore` loses all tasks if the server process restarts. For durable tasks, implement a custom <xref:ModelContextProtocol.Server.IMcpTaskStore> backed by an external store. See [Implementing a custom task store](xref:tasks#implementing-a-custom-task-store) for guidance.
 
 ### Task cancellation vs request cancellation
 
@@ -632,7 +632,7 @@ The `EventStreamStore` itself has TTL-based limits (default: 2-hour event expira
 
 ### With tasks (experimental)
 
-[Tasks](xref:tasks) are an experimental feature that enables a "call-now, fetch-later" pattern for long-running tool calls. When a client sends a task-augmented `tools/call` request, the server creates a task record in the <xref:ModelContextProtocol.IMcpTaskStore>, starts the tool handler as a fire-and-forget background task, and returns the task ID immediately — the POST response completes **before the handler starts its real work**.
+[Tasks](xref:tasks) are an experimental feature that enables a "call-now, fetch-later" pattern for long-running tool calls. When a client sends a task-augmented `tools/call` request, the server creates a task record in the <xref:ModelContextProtocol.Server.IMcpTaskStore>, starts the tool handler as a fire-and-forget background task, and returns the task ID immediately — the POST response completes **before the handler starts its real work**.
 
 This means: