fix: support SEP-2663 Tasks Extension

Problem Statement

SEP-2663 reached final status and redesigned tasks as a proper MCP extension, but mcp-go had no implementation of it.

Proposed Solution

Implement SEP-2663 with a dual-path strategy that preserves full backward compatibility with existing 2025-11-25 clients.

MCP Spec Reference

https://modelcontextprotocol.io/seps/2663-tasks-extension

Example Usage

Working on this.

// If applicable, provide sample code showing how the proposed feature would be used.
// Remove if not applicable