test: harden api contract harnesses

[rlippmann]

What changed

• add a shared API-contract harness validator for the Python conformance tests
• harden the root, audit, and grammar API-contract harnesses to reject malformed fixture schema
• keep exact export, engine member, signature, and shape-probe enforcement intact while making schema validation authoritative

Why

• fail clearly on malformed API-contract fixtures instead of silently accepting unknown or unsupported schema
• keep the conformance infrastructure aligned with the stricter fixture-contract model used elsewhere without changing runtime behavior

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)