Add support for queuing status updates in cloud language model provider (#29818)

This sets us up to display queue position information to the user, once
our language model backend is updated to support request queuing.

The JSON returned by the LLM backend will need to look like this:

```json
{"queue": {"status": "queued", "position": 1}}
{"queue": {"status": "started"}}
{"event": {"THE_UPSTREAM_MODEL_PROVIDER_EVENT": "..."}} 
```

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <git@maxdeviant.com>

crates/eval/src/instance.rs

@@ -1017,7 +1017,8 @@ pub fn response_events_to_markdown(
             }
             Ok(
                 LanguageModelCompletionEvent::UsageUpdate(_)
-                | LanguageModelCompletionEvent::StartMessage { .. },
+                | LanguageModelCompletionEvent::StartMessage { .. }
+                | LanguageModelCompletionEvent::QueueUpdate { .. },
             ) => {}
             Err(error) => {
                 flush_buffers(&mut response, &mut text_buffer, &mut thinking_buffer);
@@ -1092,6 +1093,7 @@ impl ThreadDialog {
 
                 // Skip these
                 Ok(LanguageModelCompletionEvent::UsageUpdate(_))
+                | Ok(LanguageModelCompletionEvent::QueueUpdate { .. })
                 | Ok(LanguageModelCompletionEvent::StartMessage { .. })
                 | Ok(LanguageModelCompletionEvent::Stop(_)) => {}