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/provider/google.rs

@@ -20,7 +20,7 @@ use util::ResultExt;
 use crate::{
     settings::AllLanguageModelSettings, LanguageModel, LanguageModelId, LanguageModelName,
     LanguageModelProvider, LanguageModelProviderId, LanguageModelProviderName,
-    LanguageModelProviderState, LanguageModelRequest,
+    LanguageModelProviderState, LanguageModelRequest, RateLimiter,
 };
 
 const PROVIDER_ID: &str = "google";
@@ -111,6 +111,7 @@ impl LanguageModelProvider for GoogleLanguageModelProvider {
                     model,
                     state: self.state.clone(),
                     http_client: self.http_client.clone(),
+                    rate_limiter: RateLimiter::new(4),
                 }) as Arc<dyn LanguageModel>
             })
             .collect()
@@ -120,7 +121,7 @@ impl LanguageModelProvider for GoogleLanguageModelProvider {
         self.state.read(cx).api_key.is_some()
     }
 
-    fn authenticate(&self, cx: &AppContext) -> Task<Result<()>> {
+    fn authenticate(&self, cx: &mut AppContext) -> Task<Result<()>> {
         if self.is_authenticated(cx) {
             Task::ready(Ok(()))
         } else {
@@ -153,7 +154,7 @@ impl LanguageModelProvider for GoogleLanguageModelProvider {
             .into()
     }
 
-    fn reset_credentials(&self, cx: &AppContext) -> Task<Result<()>> {
+    fn reset_credentials(&self, cx: &mut AppContext) -> Task<Result<()>> {
         let state = self.state.clone();
         let delete_credentials =
             cx.delete_credentials(&AllLanguageModelSettings::get_global(cx).google.api_url);
@@ -172,6 +173,7 @@ pub struct GoogleLanguageModel {
     model: google_ai::Model,
     state: gpui::Model<State>,
     http_client: Arc<dyn HttpClient>,
+    rate_limiter: RateLimiter,
 }
 
 impl LanguageModel for GoogleLanguageModel {
@@ -243,17 +245,17 @@ impl LanguageModel for GoogleLanguageModel {
             return futures::future::ready(Err(anyhow!("App state dropped"))).boxed();
         };
 
-        async move {
+        let future = self.rate_limiter.stream(async move {
             let api_key = api_key.ok_or_else(|| anyhow!("missing api key"))?;
             let response =
                 stream_generate_content(http_client.as_ref(), &api_url, &api_key, request);
             let events = response.await?;
             Ok(google_ai::extract_text_from_events(events).boxed())
-        }
-        .boxed()
+        });
+        async move { Ok(future.await?.boxed()) }.boxed()
     }
 
-    fn use_tool(
+    fn use_any_tool(
         &self,
         _request: LanguageModelRequest,
         _name: String,