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/copilot_chat.rs

@@ -265,13 +265,15 @@ impl LanguageModel for CopilotChatLanguageModel {
         'static,
         Result<
             BoxStream<'static, Result<LanguageModelCompletionEvent, LanguageModelCompletionError>>,
+            LanguageModelCompletionError,
         >,
     > {
         if let Some(message) = request.messages.last() {
             if message.contents_empty() {
                 const EMPTY_PROMPT_MSG: &str =
                     "Empty prompts aren't allowed. Please provide a non-empty prompt.";
-                return futures::future::ready(Err(anyhow::anyhow!(EMPTY_PROMPT_MSG))).boxed();
+                return futures::future::ready(Err(anyhow::anyhow!(EMPTY_PROMPT_MSG).into()))
+                    .boxed();
             }
 
             // Copilot Chat has a restriction that the final message must be from the user.
@@ -279,13 +281,13 @@ impl LanguageModel for CopilotChatLanguageModel {
             // and provide a more helpful error message.
             if !matches!(message.role, Role::User) {
                 const USER_ROLE_MSG: &str = "The final message must be from the user. To provide a system prompt, you must provide the system prompt followed by a user prompt.";
-                return futures::future::ready(Err(anyhow::anyhow!(USER_ROLE_MSG))).boxed();
+                return futures::future::ready(Err(anyhow::anyhow!(USER_ROLE_MSG).into())).boxed();
             }
         }
 
         let copilot_request = match into_copilot_chat(&self.model, request) {
             Ok(request) => request,
-            Err(err) => return futures::future::ready(Err(err)).boxed(),
+            Err(err) => return futures::future::ready(Err(err.into())).boxed(),
         };
         let is_streaming = copilot_request.stream;