agent: Snapshot context in user message instead of recreating it (#27967)

This makes context essentially work the same way as `read-file`,
increasing the likelihood of cache hits.

Just like with `read-file`, we'll notify the model when the user makes
an edit to one of the tracked files. In the future, we want to send a
diff instead of just a list of files, but that's an orthogonal change.


Release Notes:
- agent: Improved caching of files in context

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>

crates/assistant_eval/src/headless_assistant.rs

@@ -158,7 +158,7 @@ impl HeadlessAssistant {
                     let model_registry = LanguageModelRegistry::read_global(cx);
                     if let Some(model) = model_registry.active_model() {
                         thread.update(cx, |thread, cx| {
-                            thread.attach_tool_results(vec![], cx);
+                            thread.attach_tool_results(cx);
                             thread.send_to_model(model, RequestKind::Chat, cx);
                         });
                     } else {