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/src/terminal_inline_assistant.rs

@@ -19,7 +19,7 @@ use language_model::{
     ConfiguredModel, LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage,
     Role, report_assistant_event,
 };
-use language_model_selector::{LanguageModelSelector, LanguageModelSelectorPopoverMenu, ModelType};
+use language_model_selector::{LanguageModelSelector, LanguageModelSelectorPopoverMenu};
 use prompt_store::PromptBuilder;
 use settings::{Settings, update_settings_file};
 use std::{
@@ -749,6 +749,7 @@ impl PromptEditor {
             language_model_selector: cx.new(|cx| {
                 let fs = fs.clone();
                 LanguageModelSelector::new(
+                    |cx| LanguageModelRegistry::read_global(cx).default_model(),
                     move |model, cx| {
                         update_settings_file::<AssistantSettings>(
                             fs.clone(),
@@ -756,7 +757,6 @@ impl PromptEditor {
                             move |settings, _| settings.set_model(model.clone()),
                         );
                     },
-                    ModelType::Default,
                     window,
                     cx,
                 )