RuntimeError: Task attached to a different loop

Root cause

Background task and asyncio.Queue were initialized during FastAPI startup from a session-scoped pytest fixture, then pytest-asyncio reset the running event loop before the failing test executed. The webhook handler later awaited a queue bound to the stale loop.

Minimal fix

Make the FastAPI app/client fixture function-scoped, create async primitives only inside the active test loop, and explicitly await application startup before issuing requests. For example, replace the session-scoped AsyncClient fixture with a function-scoped fixture using LifespanManager(app) and ASGITransport, and ensure shutdown cancels the background retry worker.