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/eval/src/instance.rs

@@ -573,6 +573,7 @@ impl ExampleInstance {
             let request = LanguageModelRequest {
                 thread_id: None,
                 prompt_id: None,
+                mode: None,
                 messages: vec![LanguageModelRequestMessage {
                     role: Role::User,
                     content: vec![MessageContent::Text(to_prompt(assertion.description))],