Fix inline assistant fallback model selection (#63963)

Use the registry's default_model() accessor so inline assist can use the
same automatically selected fallback model as agent sessions when no
configured default model resolves. Keep explicit inline model selection
as the first choice.

Related to #59309.

Release Notes:

- Fixed Inline Assistant failing to select an available fallback model
when no default model is configured.

---------

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>

crates/language_model/src/registry.rs

@@ -488,7 +488,7 @@ impl LanguageModelRegistry {
 
         self.inline_assistant_model
             .clone()
-            .or_else(|| self.default_model.clone())
+            .or_else(|| self.default_model())
     }
 
     pub fn commit_message_model(&self, cx: &App) -> Option<ConfiguredModel> {
@@ -538,7 +538,7 @@ impl LanguageModelRegistry {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::fake_provider::FakeLanguageModelProvider;
+    use crate::fake_provider::{FakeLanguageModel, FakeLanguageModelProvider};
 
     #[test]
     fn selected_model_allows_slashes_in_model_id() {
@@ -701,12 +701,61 @@ mod tests {
             );
 
             assert!(registry.default_model().is_none());
+            assert!(registry.inline_assistant_model().is_none());
 
             registry.set_should_use_fallback(true);
 
             let default_model = registry.default_model().unwrap();
             assert_eq!(default_model.model.id(), model.id());
             assert_eq!(default_model.provider.id(), provider.id());
+            assert!(
+                registry
+                    .inline_assistant_model()
+                    .is_some_and(|inline_model| inline_model.is_same_as(&default_model))
+            );
+        });
+    }
+
+    #[gpui::test]
+    fn test_inline_assistant_model_precedence(cx: &mut App) {
+        let registry = cx.new(|_| LanguageModelRegistry::default());
+        let provider = Arc::new(FakeLanguageModelProvider::default());
+        let [inline_model, default_model, fallback_model] =
+            ["inline", "default", "fallback"].map(|model_id| ConfiguredModel {
+                provider: provider.clone(),
+                model: Arc::new(FakeLanguageModel::with_id_and_thinking(
+                    "fake", model_id, model_id, false,
+                )),
+            });
+
+        registry.update(cx, |registry, cx| {
+            registry.set_should_use_fallback(true);
+            registry.set_fallback_model(Some(fallback_model.clone()), cx);
+            registry.set_default_model(Some(default_model.clone()), cx);
+            registry.set_inline_assistant_model(Some(inline_model.clone()), cx);
+
+            assert!(
+                registry
+                    .inline_assistant_model()
+                    .is_some_and(|model| model.is_same_as(&inline_model))
+            );
+
+            registry.set_inline_assistant_model(None, cx);
+            assert!(
+                registry
+                    .inline_assistant_model()
+                    .is_some_and(|model| model.is_same_as(&default_model))
+            );
+
+            registry.set_default_model(None, cx);
+            assert!(
+                registry
+                    .inline_assistant_model()
+                    .is_some_and(|model| model.is_same_as(&fallback_model))
+            );
+
+            registry.set_should_use_fallback(false);
+            assert!(registry.inline_assistant_model().is_none());
         });
     }