refactor: consolidate shims to @modelcontextprotocol/core/_shims

Move runtime shims from server and client packages to just core:
- Server and client now import from @modelcontextprotocol/core/_shims
- Removed redundant shim files from server and client packages
- Updated migration docs to reference core/_shims import path

This simplifies the shim structure - there's now only one place
to define the runtime-specific validators.

Author: mattzcarey