Python asyncio Patterns

Description

Write and review correct async Python with asyncio - the single-threaded event loop and what it does and doesn't speed up, coroutines vs tasks, create_task and keeping task references, gather with return_exceptions, TaskGroup and ExceptionGroup (3.11+), timeouts with asyncio.timeout and wait_for, bounding concurrency with semaphores, producer-consumer queues with backpressure, async context managers and generators, never blocking the loop (to_thread, process pools), cancellation, and async libraries (httpx, asyncpg pools, aiofiles). Use whenever the user writes async def or await, uses asyncio, FastAPI, aiohttp, httpx or asyncpg, sees "coroutine was never awaited", "Task was destroyed but it is pending", an event loop that freezes or is slow, wants to run requests concurrently, or asks async vs threads vs processes.