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/agent/src/buffer_codegen.rs

@@ -156,8 +156,9 @@ impl BufferCodegen {
         }
 
         let primary_model = LanguageModelRegistry::read_global(cx)
-            .active_model()
-            .context("no active model")?;
+            .default_model()
+            .context("no active model")?
+            .model;
 
         for (model, alternative) in iter::once(primary_model)
             .chain(alternative_models)