UNIXSocket.aclose and _TrioSocketMixin.aclose in cancelled scope does not raise a cancelled error (does not checkpoint)

### Things to check first

- [x] I have searched the existing issues and didn't find my bug already reported there

- [x] I have checked that my bug is still present in the latest release

### AnyIO version

a8900c22b58debb7cd43a4c8d94a1905e506a797

### Python version

py 3.14

### What happened?

```py
async def test_aclose_in_cancelled_scope_raises_cancelled_exc(
self, server_sock: socket.socket, socket_path: Path
) -> None:
exc = None
stream = await connect_unix(socket_path)

with CancelScope() as scope:
scope.cancel()
try:
await stream.aclose()
except get_cancelled_exc_class() as e:
exc = e
raise

assert exc is not None
```

failed

### How can we reproduce the bug?

see above

Start with the reproduced test test_aclose_in_cancelled_scope_raises_cancelled_exc and inspect UNIXSocket.aclose and _TrioSocketMixin.aclose. Run the test to observe the failure, then verify that closing the stream in an already-cancelled scope raises get_cancelled_exc_class() as expected.

Vom Indexierungsmodell aus dem Issue-Text verfasst.