agent: Introduce ModelUsageContext (#32076)

This PR is a refactor of the existing `ModelType` in
`agent_model_selector`.

In #31848 we also need to know which context we are operating in, to
check if the configured model has image support.
In order to deduplicate the logic needed, I introduced a new type called
`ModelUsageContext` which can be used throughout the agent crate


Release Notes:

- N/A

crates/agent/src/agent_model_selector.rs

@@ -3,7 +3,7 @@ use fs::Fs;
 use gpui::{Entity, FocusHandle, SharedString};
 use picker::popover_menu::PickerPopoverMenu;
 
-use crate::Thread;
+use crate::ModelUsageContext;
 use assistant_context_editor::language_model_selector::{
     LanguageModelSelector, ToggleModelSelector, language_model_selector,
 };
@@ -12,12 +12,6 @@ use settings::update_settings_file;
 use std::sync::Arc;
 use ui::{PopoverMenuHandle, Tooltip, prelude::*};
 
-#[derive(Clone)]
-pub enum ModelType {
-    Default(Entity<Thread>),
-    InlineAssistant,
-}
-
 pub struct AgentModelSelector {
     selector: Entity<LanguageModelSelector>,
     menu_handle: PopoverMenuHandle<LanguageModelSelector>,
@@ -29,7 +23,7 @@ impl AgentModelSelector {
         fs: Arc<dyn Fs>,
         menu_handle: PopoverMenuHandle<LanguageModelSelector>,
         focus_handle: FocusHandle,
-        model_type: ModelType,
+        model_usage_context: ModelUsageContext,
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> Self {
@@ -38,19 +32,14 @@ impl AgentModelSelector {
                 let fs = fs.clone();
                 language_model_selector(
                     {
-                        let model_type = model_type.clone();
-                        move |cx| match &model_type {
-                            ModelType::Default(thread) => thread.read(cx).configured_model(),
-                            ModelType::InlineAssistant => {
-                                LanguageModelRegistry::read_global(cx).inline_assistant_model()
-                            }
-                        }
+                        let model_context = model_usage_context.clone();
+                        move |cx| model_context.configured_model(cx)
                     },
                     move |model, cx| {
                         let provider = model.provider_id().0.to_string();
                         let model_id = model.id().0.to_string();
-                        match &model_type {
-                            ModelType::Default(thread) => {
+                        match &model_usage_context {
+                            ModelUsageContext::Thread(thread) => {
                                 thread.update(cx, |thread, cx| {
                                     let registry = LanguageModelRegistry::read_global(cx);
                                     if let Some(provider) = registry.provider(&model.provider_id())
@@ -72,7 +61,7 @@ impl AgentModelSelector {
                                     },
                                 );
                             }
-                            ModelType::InlineAssistant => {
+                            ModelUsageContext::InlineAssistant => {
                                 update_settings_file::<AgentSettings>(
                                     fs.clone(),
                                     cx,