RuntimeError: Task attached to a different loop

Root cause

The FastAPI app fixture is session-scoped and calls startup once, creating app.state.dispatcher._worker_task on pytest-asyncio's initial session loop. Later pytest-asyncio resets or replaces the running event loop for the async test, so the request handler awaits a background worker task bound to the stale loop.

Minimal fix

Make the app/client async fixtures function-scoped, drive FastAPI lifespan startup inside the same loop used by each test, and cleanly await dispatcher shutdown before the loop is torn down. For example, use a function-scoped event_loop or pytest-asyncio default function scope, create the ASGITransport/AsyncClient inside an async fixture, await app.router.startup() before yielding, and await app.router.shutdown() after yielding.