Rewrite Tasks integration for SEP-2663 (remove IMcpTaskStore)

Rewrite Tasks integration for SEP-2663 (highest-impact break)

Part of #481.

SDK 2.0 removes the experimental Tasks API and replaces it with the SEP-2663 Tasks extension. This server currently depends on the removed surface:

• Program.cs: builder.Services.AddSingleton<IMcpTaskStore, InMemoryMcpTaskStore>();
• DotNetCliTools.Core.cs: private readonly IMcpTaskStore? _taskStore; (constructor-injected)
• DotNetCliTools.Misc.cs: AsyncTasks = _taskStore != null in ServerFeatureSupport

Removed types include McpTask, McpTaskMetadata, McpTasksCapability, tasks/result, tasks/list. Replacements: CreateTaskResult, ResultOrCreatedTask<T>, GetTaskResult, tasks/get, tasks/status, tasks/cancel, plus the new input_required state.

Tasks

• Remove IMcpTaskStore/InMemoryMcpTaskStore registration and the _taskStore field/parameter.
• Decide: migrate long-running ops (build/test/publish/restore) to the new Tasks extension, or temporarily drop async-task support.
• If migrating, adopt ResultOrCreatedTask<T> and the io.modelcontextprotocol/tasks extension; wire tasks/get/tasks/status/tasks/cancel.
• Update AsyncTasks capability reporting in ServerFeatureSupport.

Acceptance

• Project compiles against 2.0 with a clear, documented async-task story (migrated or explicitly dropped).

Start with the IMcpTaskStore registration in Program.cs, then inspect the _taskStore field and constructor parameter in DotNetCliTools.Core.cs and AsyncTasks reporting in DotNetCliTools.Misc.cs. Build the project against SDK 2.0 and determine whether long-running operations will migrate to the Tasks extension or async-task support will be dropped. Done means the project compiles and the async-task behavior is clearly documented.