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/rules_library/src/rules_library.rs

@@ -922,6 +922,7 @@ impl RulesLibrary {
                                 LanguageModelRequest {
                                     thread_id: None,
                                     prompt_id: None,
+                                    mode: None,
                                     messages: vec![LanguageModelRequestMessage {
                                         role: Role::System,
                                         content: vec![body.to_string().into()],