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/buffer_codegen.rs

@@ -460,6 +460,7 @@ impl CodegenAlternative {
             LanguageModelRequest {
                 thread_id: None,
                 prompt_id: None,
+                mode: None,
                 tools: Vec::new(),
                 stop: Vec::new(),
                 temperature: None,