RuntimeError: Task attached to a different loop

Root cause

Background task and asyncio.Queue were initialized during a session-scoped FastAPI startup fixture, then pytest-asyncio reset the event loop before the function-scoped test executed. Starlette's BackgroundTask attempted to await queue.put() on a Future bound to the stale loop.

Minimal fix

Change the FastAPI app/client fixture to function scope, create async resources inside the active pytest-asyncio event loop, and explicitly await application startup before yielding the AsyncClient. Avoid constructing the background delivery queue at import time or in a session-scoped fixture.