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/inline_prompt_editor.rs

@@ -1,4 +1,4 @@
-use crate::assistant_model_selector::AssistantModelSelector;
+use crate::assistant_model_selector::{AssistantModelSelector, ModelType};
 use crate::buffer_codegen::BufferCodegen;
 use crate::context_picker::ContextPicker;
 use crate::context_store::ContextStore;
@@ -20,7 +20,7 @@ use gpui::{
     Focusable, FontWeight, Subscription, TextStyle, WeakEntity, Window, anchored, deferred, point,
 };
 use language_model::{LanguageModel, LanguageModelRegistry};
-use language_model_selector::{ModelType, ToggleModelSelector};
+use language_model_selector::ToggleModelSelector;
 use parking_lot::Mutex;
 use settings::Settings;
 use std::cmp;