When the Tokio runtime is dropped during Zed shutdown, it drives all
spawned tasks to cancellation by polling them one last time. The
`wasmtime-wasi` crate wraps child task `JoinHandle`s in an
`AbortOnDropJoinHandle` whose `Future::poll` calls `.expect("child task
panicked")` on the join result — but Tokio can also return
`JoinError::Cancelled` (not just panics), causing the expect to panic
with:

```
child task panicked: JoinError::Cancelled(Id(45))
```

This showed up as Sentry issue ZED-69A (106 events, 85 users, 100%
Windows, stable channel).

The fix is to explicitly call `shutdown_background()` on the owned Tokio
runtime during `GlobalTokio::drop`, which immediately drops all spawned
tasks without polling them again, avoiding the wasmtime-wasi panic path
entirely.

Release Notes:

- Fixed a crash on exit caused by the Tokio runtime shutdown triggering
a panic in extension WASI tasks (Windows).