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_model/src/fake_provider.rs

@@ -1,6 +1,6 @@
 use crate::{
-    LanguageModel, LanguageModelCompletionEvent, LanguageModelId, LanguageModelName,
-    LanguageModelProvider, LanguageModelProviderId, LanguageModelProviderName,
+    AuthenticateError, LanguageModel, LanguageModelCompletionEvent, LanguageModelId,
+    LanguageModelName, LanguageModelProvider, LanguageModelProviderId, LanguageModelProviderName,
     LanguageModelProviderState, LanguageModelRequest,
 };
 use futures::{channel::mpsc, future::BoxFuture, stream::BoxStream, FutureExt, StreamExt};
@@ -54,7 +54,7 @@ impl LanguageModelProvider for FakeLanguageModelProvider {
         true
     }
 
-    fn authenticate(&self, _: &mut App) -> Task<Result<()>> {
+    fn authenticate(&self, _: &mut App) -> Task<Result<(), AuthenticateError>> {
         Task::ready(Ok(()))
     }