RuntimeError: Task attached to a different loop

Root cause

A session-scoped FastAPI app fixture calls app startup once and creates app.state.delivery_queue plus a background dispatcher task on the initial pytest-asyncio event loop. Later tests run after pytest-asyncio resets or replaces the running loop, so the request handler awaits Queue.put() on a queue/future bound to the stale loop.

Minimal fix

Make the async client/app lifecycle fixture function-scoped, ensure pytest-asyncio uses a function-scoped event loop, and explicitly await FastAPI lifespan startup/shutdown inside the fixture so background tasks and asyncio.Queue are created on the same loop as each test. For example, use LifespanManager(app) around AsyncClient with ASGITransport instead of starting the app at import/session scope.