language_model: Return AuthenticateErrors from LanguageModelProvider::authenticate (#25126)

This PR updates the `LanguageModelProvider::authenticate` method to
return an `AuthenticateError` instead of an `anyhow::Error`.

This allows us to model the "credentials not found" state explicitly as
`AuthenticateError::CredentialsNotFound`, which enables the caller to
check for this state and act accordingly.

Planning to use this in #25123 to silence errors about missing
credentials when authenticating providers in the background.

Release Notes:

- N/A

crates/language_models/src/provider/lmstudio.rs

@@ -2,7 +2,7 @@ use anyhow::{anyhow, Result};
 use futures::{future::BoxFuture, stream::BoxStream, FutureExt, StreamExt};
 use gpui::{AnyView, App, AsyncApp, Context, Subscription, Task};
 use http_client::HttpClient;
-use language_model::LanguageModelCompletionEvent;
+use language_model::{AuthenticateError, LanguageModelCompletionEvent};
 use language_model::{
     LanguageModel, LanguageModelId, LanguageModelName, LanguageModelProvider,
     LanguageModelProviderId, LanguageModelProviderName, LanguageModelProviderState,
@@ -90,12 +90,13 @@ impl State {
         self.fetch_model_task.replace(task);
     }
 
-    fn authenticate(&mut self, cx: &mut Context<Self>) -> Task<Result<()>> {
+    fn authenticate(&mut self, cx: &mut Context<Self>) -> Task<Result<(), AuthenticateError>> {
         if self.is_authenticated() {
-            Task::ready(Ok(()))
-        } else {
-            self.fetch_models(cx)
+            return Task::ready(Ok(()));
         }
+
+        let fetch_models_task = self.fetch_models(cx);
+        cx.spawn(|_this, _cx| async move { Ok(fetch_models_task.await?) })
     }
 }
 
@@ -201,7 +202,7 @@ impl LanguageModelProvider for LmStudioLanguageModelProvider {
         self.state.read(cx).is_authenticated()
     }
 
-    fn authenticate(&self, cx: &mut App) -> Task<Result<()>> {
+    fn authenticate(&self, cx: &mut App) -> Task<Result<(), AuthenticateError>> {
         self.state.update(cx, |state, cx| state.authenticate(cx))
     }