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/assistant_context_editor/src/context_editor.rs

@@ -39,7 +39,7 @@ use language_model::{
     Role,
 };
 use language_model_selector::{
-    LanguageModelSelector, LanguageModelSelectorPopoverMenu, ModelType, ToggleModelSelector,
+    LanguageModelSelector, LanguageModelSelectorPopoverMenu, ToggleModelSelector,
 };
 use multi_buffer::MultiBufferRow;
 use picker::Picker;
@@ -291,6 +291,7 @@ impl ContextEditor {
             dragged_file_worktrees: Vec::new(),
             language_model_selector: cx.new(|cx| {
                 LanguageModelSelector::new(
+                    |cx| LanguageModelRegistry::read_global(cx).default_model(),
                     move |model, cx| {
                         update_settings_file::<AssistantSettings>(
                             fs.clone(),
@@ -298,7 +299,6 @@ impl ContextEditor {
                             move |settings, _| settings.set_model(model.clone()),
                         );
                     },
-                    ModelType::Default,
                     window,
                     cx,
                 )