feat(mcp): support modern task execution

[carldebilly]

Summary

Adds modern MCP Tasks support for Repl commands marked .LongRunning().

Behavior

• Uses the official ModelContextProtocol.Extensions.Tasks 2.2.0 extension.
• A modern client that negotiates io.modelcontextprotocol/tasks receives a task handle for a long-running command and can poll or cancel it.
• Commands without .LongRunning() remain synchronous.
• Clients without Tasks support, including legacy clients, receive the existing synchronous tools/call response.
• Uses an in-memory task store by default, suitable for stdio; ReplMcpServerOptions.TaskStore allows a durable shared store for HTTP or restart-resilient deployments.

Documentation

Documents the modern-only protocol, the default store, and the current SDK limitation: task-backed MCP tools cannot compose with MRTR/interactive flows yet.

Validation

• C# integration tests using the official SDK client cover task creation and polling, synchronous behavior, legacy fallback, and cancellation.
• MSTest runs the official MCP Inspector 2.2.0 against the real stdio sample, covering normal resources and the long-running synchronous fallback.
• MCP suite: 234 passing, 2 Inspector tests opt-in; Inspector run: 2/2 passing.

Closes #72.