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/git_ui/src/git_panel.rs

@@ -1746,6 +1746,7 @@ impl GitPanel {
                 let request = LanguageModelRequest {
                     thread_id: None,
                     prompt_id: None,
+                    mode: None,
                     messages: vec![LanguageModelRequestMessage {
                         role: Role::User,
                         content: vec![content.into()],