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_models/src/provider/open_router.rs

@@ -367,6 +367,7 @@ impl LanguageModel for OpenRouterLanguageModel {
                 'static,
                 Result<LanguageModelCompletionEvent, LanguageModelCompletionError>,
             >,
+            LanguageModelCompletionError,
         >,
     > {
         let request = into_open_router(request, &self.model, self.max_output_tokens());