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/language_model/src/language_model.rs

@@ -64,9 +64,17 @@ pub struct LanguageModelCacheConfiguration {
     pub min_total_token: usize,
 }
 
+#[derive(Debug, PartialEq, Clone, Copy, Serialize, Deserialize)]
+#[serde(tag = "status", rename_all = "snake_case")]
+pub enum QueueState {
+    Queued { position: usize },
+    Started,
+}
+
 /// A completion event from a language model.
 #[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
 pub enum LanguageModelCompletionEvent {
+    QueueUpdate(QueueState),
     Stop(StopReason),
     Text(String),
     Thinking {
@@ -349,6 +357,7 @@ pub trait LanguageModel: Send + Sync {
                         let last_token_usage = last_token_usage.clone();
                         async move {
                             match result {
+                                Ok(LanguageModelCompletionEvent::QueueUpdate { .. }) => None,
                                 Ok(LanguageModelCompletionEvent::StartMessage { .. }) => None,
                                 Ok(LanguageModelCompletionEvent::Text(text)) => Some(Ok(text)),
                                 Ok(LanguageModelCompletionEvent::Thinking { .. }) => None,