Slice 2 of the management-endpoints behavior-pinning effort. New top-level dir
tests/proxy_behavior/management/ outside every existing pytest glob.
conftest.py initialises the proxy app once per session against the DATABASE_URL
the harness boots Postgres at, wraps it in httpx.AsyncClient via in-process
ASGITransport. The one smoke test asserts /health/liveliness returns 200, which
exercises the full FastAPI middleware stack against a real app — no mocks.
Plan: https://www.notion.so/36643b8acdab8128a581ced0f6a4744d
