Remove Tasks (SEP-1686) from the SDK

Tasks have been removed from the MCP specification and will return as a
separate extension in a future release. This removes all task-related
code, types, examples, tests, and documentation from the SDK.

Removed:
- All Task* types, capability fields, Tool.execution, and union members
  from mcp.types
- mcp.{client,server,shared}.experimental packages
- mcp.server.lowlevel.experimental and mcp.shared.response_router
- experimental properties on ClientSession, ServerSession, Server, and
  the experimental field on ServerRequestContext
- ServerSession._build_*_request helpers, send_message, and the tasks
  capability check (task-only consumers)
- BaseSession.add_response_router and the router-dispatch loop in
  _handle_response (the method extraction itself is kept)
- tests/experimental/ and the tasks:* entries in
  tests/interaction/_requirements.py
- examples/{clients,servers}/simple-task* and their uv.lock workspace
  members
- docs/experimental/ and the mkdocs nav section

Kept:
- CancelledNotificationParams.request_id stays optional to match the
  draft schema; only the docstring was updated
- server/validation.py (non-task sampling-validation refactor)
- _handle_response extraction in BaseSession

Added a test for ServerSession.check_client_capability since the
experimental task tests were its only coverage.

Author: maxisbey

README.v2.md

@@ -2489,7 +2489,6 @@ MCP servers declare capabilities during initialization:
 ## Documentation
 
 - [API Reference](https://modelcontextprotocol.github.io/python-sdk/api/)
-- [Experimental Features (Tasks)](https://modelcontextprotocol.github.io/python-sdk/experimental/tasks/)
 - [Model Context Protocol documentation](https://modelcontextprotocol.io)
 - [Model Context Protocol specification](https://modelcontextprotocol.io/specification/latest)
 - [Officially supported servers](https://github.com/modelcontextprotocol/servers)