Restructure workflow step resolution and fix inserting newlines (#15720)

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/language_model/src/language_model.rs

@@ -75,6 +75,11 @@ pub trait LanguageModel: Send + Sync {
         schema: serde_json::Value,
         cx: &AsyncAppContext,
     ) -> BoxFuture<'static, Result<serde_json::Value>>;
+
+    #[cfg(any(test, feature = "test-support"))]
+    fn as_fake(&self) -> &provider::fake::FakeLanguageModel {
+        unimplemented!()
+    }
 }
 
 impl dyn LanguageModel {