evals: Configurable number of max dialog turns (#31680)

Release Notes:

- N/A

crates/eval/src/examples/planets.rs

@@ -19,6 +19,7 @@ impl Example for Planets {
             max_assertions: None,
             profile_id: AgentProfileId::default(),
             existing_thread_json: None,
+            max_turns: None,
         }
     }