cloud_llm_client: Make default_model and default_fast_model optional (#38288)

This PR makes the `default_model` and `default_fast_model` fields
optional on the `ListModelsResponse`.

Release Notes:

- N/A

crates/language_models/src/provider/cloud.rs

@@ -215,11 +215,21 @@ impl State {
 
         self.default_model = models
             .iter()
-            .find(|model| model.id == response.default_model)
+            .find(|model| {
+                response
+                    .default_model
+                    .as_ref()
+                    .is_some_and(|default_model_id| &model.id == default_model_id)
+            })
             .cloned();
         self.default_fast_model = models
             .iter()
-            .find(|model| model.id == response.default_fast_model)
+            .find(|model| {
+                response
+                    .default_fast_model
+                    .as_ref()
+                    .is_some_and(|default_fast_model_id| &model.id == default_fast_model_id)
+            })
             .cloned();
         self.recommended_models = response
             .recommended_models