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/agent/src/active_thread.rs

@@ -1293,6 +1293,7 @@ impl ActiveThread {
                 let request = language_model::LanguageModelRequest {
                     thread_id: None,
                     prompt_id: None,
+                    mode: None,
                     messages: vec![request_message],
                     tools: vec![],
                     stop: vec![],