Associate each thread with a model (#29573)

This PR makes it possible to use different LLM models in the agent
panels of two different projects, simultaneously. It also properly
restores a thread's original model when restoring it from the history,
rather than having it use the default model. As before, newly-created
threads will use the current default model.

Release Notes:

- Enabled different project windows to use different models in the agent
panel
- Enhanced the agent panel so that when revisiting old threads, their
original model will be used.

---------

Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/agent/src/agent_diff.rs

@@ -951,6 +951,7 @@ mod tests {
             ThemeSettings::register(cx);
             ContextServerSettings::register(cx);
             EditorSettings::register(cx);
+            language_model::init_settings(cx);
         });
 
         let fs = FakeFs::new(cx.executor());