Add token usage to LanguageModelTextStream (#27490)

Release Notes:

- N/A

---------

Co-authored-by: Michael Sloan <michael@zed.dev>

crates/assistant/src/inline_assistant.rs

@@ -3712,7 +3712,7 @@ mod tests {
         language_settings, tree_sitter_rust, Buffer, Language, LanguageConfig, LanguageMatcher,
         Point,
     };
-    use language_model::LanguageModelRegistry;
+    use language_model::{LanguageModelRegistry, TokenUsage};
     use rand::prelude::*;
     use serde::Serialize;
     use settings::SettingsStore;
@@ -4091,6 +4091,7 @@ mod tests {
                 future::ready(Ok(LanguageModelTextStream {
                     message_id: None,
                     stream: chunks_rx.map(Ok).boxed(),
+                    last_token_usage: Arc::new(Mutex::new(TokenUsage::default())),
                 })),
                 cx,
             );