ai: Separate model settings for each feature (#28088)

Closes: https://github.com/zed-industries/zed/issues/20582

Allows users to select a specific model for each AI-powered feature:
- Agent panel
- Inline assistant
- Thread summarization
- Commit message generation

If unspecified for a given feature, it will use the `default_model`
setting.

Release Notes:

- Added support for configuring a specific model for each AI-powered
feature

---------

Co-authored-by: Danilo Leal <daniloleal09@gmail.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>

crates/assistant_eval/src/headless_assistant.rs

@@ -156,10 +156,10 @@ impl HeadlessAssistant {
                 }
                 if thread.read(cx).all_tools_finished() {
                     let model_registry = LanguageModelRegistry::read_global(cx);
-                    if let Some(model) = model_registry.active_model() {
+                    if let Some(model) = model_registry.default_model() {
                         thread.update(cx, |thread, cx| {
                             thread.attach_tool_results(cx);
-                            thread.send_to_model(model, RequestKind::Chat, cx);
+                            thread.send_to_model(model.model, RequestKind::Chat, cx);
                         });
                     } else {
                         println!(