Add initial implementation of evaluating changes generated by the assistant (#26799)

Release Notes:

- N/A

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>
Co-authored-by: Thomas <thomas@zed.dev>

crates/assistant2/src/active_thread.rs

@@ -298,6 +298,7 @@ impl ActiveThread {
             ThreadEvent::StreamedCompletion | ThreadEvent::SummaryChanged => {
                 self.save_thread(cx);
             }
+            ThreadEvent::DoneStreaming => {}
             ThreadEvent::StreamedAssistantText(message_id, text) => {
                 if let Some(markdown) = self.rendered_messages_by_id.get_mut(&message_id) {
                     markdown.update(cx, |markdown, cx| {