fix(tasks): clean up the singleflight cache on cancellation

`_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.

src/task_scheduler.py

@@ -97,19 +97,30 @@ async def _cached(key: Tuple, ttl: float, fetch: Callable[[], Awaitable[Any]]) -
             pending = fut
             owner = True
     if not owner:
-        return await pending
+        # A cancelled waiter must not cancel the shared Future for the owner
+        # and every other waiter.
+        return await asyncio.shield(pending)
     try:
         val = await fetch()
         async with _shared_cache_lock:
             _shared_cache[key] = (time.monotonic() + ttl, val)
-            _shared_cache_pending.pop(key, None)
         pending.set_result(val)
         return val
+    except asyncio.CancelledError:
+        # Cancellation is a BaseException on supported Python versions, so it
+        # bypasses the Exception handler below. Wake all current waiters while
+        # allowing a later caller to retry the fetch.
+        pending.cancel()
+        raise
     except Exception as e:
-        async with _shared_cache_lock:
-            _shared_cache_pending.pop(key, None)
         pending.set_exception(e)
         raise
+    finally:
+        # Keep this cleanup synchronous so a second cancellation cannot
+        # interrupt it and leave a permanently pending Future behind. All
+        # access runs on the scheduler's event-loop thread.
+        if _shared_cache_pending.get(key) is pending:
+            _shared_cache_pending.pop(key, None)
 
 
 def compute_next_run(schedule: str, scheduled_time: str,