/auto (#16696)

Add `/auto` behind a feature flag that's disabled for now, even for
staff.

We've decided on a different design for context inference, but there are
parts of /auto that will be useful for that, so we want them in the code
base even if they're unused for now.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/language_model/src/registry.rs

@@ -159,11 +159,13 @@ impl LanguageModelRegistry {
         providers
     }
 
-    pub fn available_models(&self, cx: &AppContext) -> Vec<Arc<dyn LanguageModel>> {
+    pub fn available_models<'a>(
+        &'a self,
+        cx: &'a AppContext,
+    ) -> impl Iterator<Item = Arc<dyn LanguageModel>> + 'a {
         self.providers
             .values()
             .flat_map(|provider| provider.provided_models(cx))
-            .collect()
     }
 
     pub fn provider(&self, id: &LanguageModelProviderId) -> Option<Arc<dyn LanguageModelProvider>> {