assistant: Add support for displaying billing-related errors (#19082)

This PR adds support to the assistant for display billing-related
errors.

Pulling this out of #19081 to make it easier to cherry-pick.

Release Notes:

- N/A

Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>

crates/language_model/src/registry.rs

@@ -1,3 +1,4 @@
+use crate::provider::cloud::RefreshLlmTokenListener;
 use crate::{
     provider::{
         anthropic::AnthropicLanguageModelProvider, cloud::CloudLanguageModelProvider,
@@ -30,6 +31,8 @@ fn register_language_model_providers(
 ) {
     use feature_flags::FeatureFlagAppExt;
 
+    RefreshLlmTokenListener::register(client.clone(), cx);
+
     registry.register_provider(
         AnthropicLanguageModelProvider::new(client.http_client(), cx),
         cx,