agent: Fix summarization model being cleared by unrelated registry events (#52080)

## Context

We were seeing lots of pending title generation, which should only
happen if we don't have a summarization model.

`handle_models_updated_event` unconditionally overwrote the thread's
summarization model on every registry event, even with `None`. 

We should only setting if explicitly changed by the user or we haven't
set it yet, and only if we actually have one.

It is hard to reproduce this issue.. but I don't think this code was
right in the first place anyway.


## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

crates/agent/src/agent.rs

@@ -727,7 +727,7 @@ impl NativeAgent {
     fn handle_models_updated_event(
         &mut self,
         _registry: Entity<LanguageModelRegistry>,
-        _event: &language_model::Event,
+        event: &language_model::Event,
         cx: &mut Context<Self>,
     ) {
         self.models.refresh_list(cx);
@@ -744,7 +744,13 @@ impl NativeAgent {
                     thread.set_model(model, cx);
                     cx.notify();
                 }
-                thread.set_summarization_model(summarization_model.clone(), cx);
+                if let Some(model) = summarization_model.clone() {
+                    if thread.summarization_model().is_none()
+                        || matches!(event, language_model::Event::ThreadSummaryModelChanged)
+                    {
+                        thread.set_summarization_model(Some(model), cx);
+                    }
+                }
             });
         }
     }
@@ -2456,6 +2462,61 @@ mod internal_tests {
         });
     }
 
+    #[gpui::test]
+    async fn test_summarization_model_survives_transient_registry_clearing(
+        cx: &mut TestAppContext,
+    ) {
+        init_test(cx);
+        let fs = FakeFs::new(cx.executor());
+        fs.insert_tree("/", json!({ "a": {} })).await;
+        let project = Project::test(fs.clone(), [], cx).await;
+
+        let thread_store = cx.new(|cx| ThreadStore::new(cx));
+        let agent =
+            cx.update(|cx| NativeAgent::new(thread_store, Templates::new(), None, fs.clone(), cx));
+        let connection = Rc::new(NativeAgentConnection(agent.clone()));
+
+        let acp_thread = cx
+            .update(|cx| {
+                connection.clone().new_session(
+                    project.clone(),
+                    PathList::new(&[Path::new("/a")]),
+                    cx,
+                )
+            })
+            .await
+            .unwrap();
+        let session_id = acp_thread.read_with(cx, |thread, _| thread.session_id().clone());
+
+        let thread = agent.read_with(cx, |agent, _| {
+            agent.sessions.get(&session_id).unwrap().thread.clone()
+        });
+
+        thread.read_with(cx, |thread, _| {
+            assert!(
+                thread.summarization_model().is_some(),
+                "session should have a summarization model from the test registry"
+            );
+        });
+
+        // Simulate what happens during a provider blip:
+        // update_active_language_model_from_settings calls set_default_model(None)
+        // when it can't resolve the model, clearing all fallbacks.
+        cx.update(|cx| {
+            LanguageModelRegistry::global(cx).update(cx, |registry, cx| {
+                registry.set_default_model(None, cx);
+            });
+        });
+        cx.run_until_parked();
+
+        thread.read_with(cx, |thread, _| {
+            assert!(
+                thread.summarization_model().is_some(),
+                "summarization model should survive a transient default model clearing"
+            );
+        });
+    }
+
     #[gpui::test]
     async fn test_loaded_thread_preserves_thinking_enabled(cx: &mut TestAppContext) {
         init_test(cx);