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/language_model/src/registry.rs

@@ -188,7 +188,7 @@ impl LanguageModelRegistry {
             .collect::<Vec<_>>();
     }
 
-    fn select_model(
+    pub fn select_model(
         &mut self,
         selected_model: &SelectedModel,
         cx: &mut Context<Self>,