Extract protocol types into a standalone mcp-types package

[Kludex]

Summary

Converts the repository into a uv workspace mirroring pydantic/httpx2, introducing a standalone mcp-types package for the protocol types.

• Root pyproject.toml becomes a pure coordinator ([tool.uv] package = false, no [project]); shared tool config (ruff, pyright, pytest, coverage) stays at root.
• Two distributable packages, each nested under src/<pkg>/<pkg>/ with its own pyproject.toml:
  • src/mcp/mcp/ → mcp
  • src/mcp-types/mcp_types/ → mcp-types (import name mcp_types, with py.typed)
• The protocol types (_types.py, jsonrpc.py) move into mcp_types, which only depends on pydantic and typing-extensions. mcp depends on mcp-types=={{ version }} via uv-dynamic-versioning, so installing mcp continues to pull the types in.
• The mcp.types module path is removed; all imports now use mcp_types. The curated re-exports on the top-level mcp package (from mcp import Tool) are unchanged.
• Migration guide and README.v2.md snippets updated.

Verification

• uv lock / uv sync build both packages from their new locations
• ruff check + ruff format --check: clean
• pyright: 0 errors
• ./scripts/test: 1750 passed, 100% coverage, strict-no-cover clean

AI Disclaimer

This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.