Remove Tasks (SEP-1686) from the SDK

[maxisbey]

Tasks (SEP-1686) have been removed from the MCP specification (modelcontextprotocol/modelcontextprotocol#2663) and will return as a separate extension. This removes all task-related code, types, examples, tests, and documentation from the SDK.

Motivation and Context

The original Tasks SEP that landed in the 2025-11-25 spec has been pulled from the draft schema and is moving to an extension. Keeping the experimental implementation around would mean maintaining ~13k lines against a moving target that no longer matches the spec, so this strips it entirely. When the extension lands it can be re-added against the new shape.

How Has This Been Tested?

• ./scripts/test — full suite, 100% coverage, strict-no-cover clean
• pyright, ruff, pre-commit hooks all pass
• uv lock regenerated to drop the four simple-task* workspace members

Breaking Changes

Yes (v2 only — main is already a breaking-change branch). Documented in docs/migration.md:

• All Task* / *TasksCapability types, ToolExecution, and Tool.execution removed from mcp.types
• RequestParams.task, ClientCapabilities.tasks, ServerCapabilities.tasks removed
• mcp.{client,server,shared}.experimental and mcp.server.lowlevel.experimental packages removed
• experimental properties removed from ClientSession / ServerSession / Server; experimental field removed from ServerRequestContext
• BaseSession.add_response_router and mcp.shared.response_router removed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

A few non-obvious decisions:

• CancelledNotificationParams.request_id stays RequestId | None. The Tasks PR made it optional, but the draft schema kept it that way after removing tasks, so only the task-referencing docstring was changed.
• BaseSession._handle_response is kept (with the router-dispatch loop stripped). The extraction from _receive_loop plus the null-ID error handling is a standalone improvement that doesn't depend on tasks.
• server/validation.py is kept. It's a non-task refactor of sampling-tools validation that SEP-1686: Tasks #1645 happened to extract; only the "task-augmented" wording in the module docstring was dropped.
• ResponseRouter, ServerSession.send_message, and the _build_*_request helpers are deleted rather than kept as "generic plumbing" — they had zero non-task callers, and keeping them would have required either pragmas or synthetic tests for the 100% coverage requirement.
• Added tests/server/test_session.py::test_check_client_capability because the experimental task tests were the only thing exercising that public method.

_{AI Disclaimer}