Track cumulative token usage in assistant2 when using anthropic API (#26738)

Release Notes:

- N/A

crates/assistant_context_editor/src/context.rs

@@ -2254,6 +2254,7 @@ impl AssistantContext {
                                         );
                                     }
                                     LanguageModelCompletionEvent::ToolUse(_) => {}
+                                    LanguageModelCompletionEvent::UsageUpdate(_) => {}
                                 }
                             });