Vendored protocol schemas

JSON Schema files for each protocol version the SDK has a wire-shape surface package for, vendored from the spec repository at the commit recorded in PINNED.json. scripts/gen_surface_types.py reads these to regenerate src/mcp/types/v<version>/__init__.py; CI runs the generator with --check.

To bump: drop the new schema.json here as <protocol-version>.json, update the matching entry in PINNED.json (commit + sha256), and run uv run --frozen --group codegen python scripts/gen_surface_types.py.