Allow customization of the model used for tool calling (#15479)

We also eliminate the `completion` crate and moved its logic into
`LanguageModelRegistry`.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/language_model/src/rate_limiter.rs

@@ -0,0 +1,70 @@
+use anyhow::Result;
+use futures::Stream;
+use smol::lock::{Semaphore, SemaphoreGuardArc};
+use std::{
+    future::Future,
+    pin::Pin,
+    sync::Arc,
+    task::{Context, Poll},
+};
+
+#[derive(Clone)]
+pub struct RateLimiter {
+    semaphore: Arc<Semaphore>,
+}
+
+pub struct RateLimitGuard<T> {
+    inner: T,
+    _guard: SemaphoreGuardArc,
+}
+
+impl<T> Stream for RateLimitGuard<T>
+where
+    T: Stream,
+{
+    type Item = T::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Option<Self::Item>> {
+        unsafe { Pin::map_unchecked_mut(self, |this| &mut this.inner).poll_next(cx) }
+    }
+}
+
+impl RateLimiter {
+    pub fn new(limit: usize) -> Self {
+        Self {
+            semaphore: Arc::new(Semaphore::new(limit)),
+        }
+    }
+
+    pub fn run<'a, Fut, T>(&self, future: Fut) -> impl 'a + Future<Output = Result<T>>
+    where
+        Fut: 'a + Future<Output = Result<T>>,
+    {
+        let guard = self.semaphore.acquire_arc();
+        async move {
+            let guard = guard.await;
+            let result = future.await?;
+            drop(guard);
+            Ok(result)
+        }
+    }
+
+    pub fn stream<'a, Fut, T>(
+        &self,
+        future: Fut,
+    ) -> impl 'a + Future<Output = Result<impl Stream<Item = T::Item>>>
+    where
+        Fut: 'a + Future<Output = Result<T>>,
+        T: Stream,
+    {
+        let guard = self.semaphore.acquire_arc();
+        async move {
+            let guard = guard.await;
+            let inner = future.await?;
+            Ok(RateLimitGuard {
+                inner,
+                _guard: guard,
+            })
+        }
+    }
+}