More resilient eval (#32257)

Bubbles up rate limit information so that we can retry after a certain
duration if needed higher up in the stack.

Also caps the number of concurrent evals running at once to also help.

Release Notes:

- N/A

crates/language_model/src/fake_provider.rs

@@ -185,6 +185,7 @@ impl LanguageModel for FakeLanguageModel {
         'static,
         Result<
             BoxStream<'static, Result<LanguageModelCompletionEvent, LanguageModelCompletionError>>,
+            LanguageModelCompletionError,
         >,
     > {
         let (tx, rx) = mpsc::unbounded();