Make the gpui_tokio crate generic over the context it spawns (#23995)

Part of  #21092

Makes `Tokio::spawn` generic over any `AppContext`.

Also removes a stray `model_context` I missed

Release Notes:

- N/A

crates/gpui_tokio/src/gpui_tokio.rs

@@ -1,6 +1,6 @@
 use std::future::Future;
 
-use gpui::{App, Global, ReadGlobal, Task};
+use gpui::{App, AppContext, Global, ReadGlobal, Task};
 use tokio::task::JoinError;
 use util::defer;
 
@@ -32,20 +32,23 @@ pub struct Tokio {}
 impl Tokio {
     /// Spawns the given future on Tokio's thread pool, and returns it via a GPUI task
     /// Note that the Tokio task will be cancelled if the GPUI task is dropped
-    pub fn spawn<Fut, R>(cx: &mut App, f: Fut) -> Task<Result<R, JoinError>>
+    pub fn spawn<C, Fut, R>(cx: &mut C, f: Fut) -> C::Result<Task<Result<R, JoinError>>>
     where
+        C: AppContext,
         Fut: Future<Output = R> + Send + 'static,
         R: Send + 'static,
     {
-        let join_handle = GlobalTokio::global(cx).runtime.spawn(f);
-        let abort_handle = join_handle.abort_handle();
-        let cancel = defer(move || {
-            abort_handle.abort();
-        });
-        cx.background_executor().spawn(async move {
-            let result = join_handle.await;
-            drop(cancel);
-            result
+        cx.read_global(|tokio: &GlobalTokio, cx| {
+            let join_handle = tokio.runtime.spawn(f);
+            let abort_handle = join_handle.abort_handle();
+            let cancel = defer(move || {
+                abort_handle.abort();
+            });
+            cx.background_spawn(async move {
+                let result = join_handle.await;
+                drop(cancel);
+                result
+            })
         })
     }