agent: Add "max mode" toggle (#29549)

This PR adds a "max mode" toggle to the Agent panel, for models that
support it.

Only visible to folks in the `new-billing` feature flag.

Icon is just a placeholder.

Release Notes:

- N/A

crates/semantic_index/src/summary_index.rs

@@ -559,6 +559,7 @@ impl SummaryIndex {
         let request = LanguageModelRequest {
             thread_id: None,
             prompt_id: None,
+            mode: None,
             messages: vec![LanguageModelRequestMessage {
                 role: Role::User,
                 content: vec![prompt.into()],