`_cached` deduplicates the scheduler's outbound fetches — Miniflux unread
counts and MCP tool snapshots — by parking every concurrent caller on one
shared Future. Two cancellation paths left that Future stranded.
A waiter awaited the shared Future directly, so cancelling the waiter
cancelled the Future the owner and every other waiter were using. It now
awaits through `asyncio.shield`.
The owner removed its pending entry inside the success and `except Exception`
branches. `CancelledError` is a `BaseException`, so it took neither: the key
stayed in `_shared_cache_pending` pointing at a Future nobody would ever
resolve, and every later caller for that key waited forever. Cleanup moves to
a `finally` that is synchronous on purpose, and the owner cancels its own
Future so current waiters wake while a later caller can still retry.
Ported from public `dev` (`ce04dc1d`, #6174 upstream), with its test.
