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/assistant_context_editor/src/context.rs

@@ -2559,6 +2559,7 @@ impl AssistantContext {
         let mut completion_request = LanguageModelRequest {
             thread_id: None,
             prompt_id: None,
+            mode: None,
             messages: Vec::new(),
             tools: Vec::new(),
             stop: Vec::new(),