Apply suggestions from code review

Author: miguelg719

evals/evaluator.ts

@@ -38,11 +38,11 @@ export class Evaluator {
 
   constructor(
     stagehand: Stagehand,
-    _modelName?: AvailableModel,
+    modelName?: AvailableModel,
     modelClientOptions?: ClientOptions,
   ) {
     this.stagehand = stagehand;
-    this.modelName = "google/gemini-2.5-flash";
+    this.modelName = modelName || "google/gemini-2.5-flash";
     this.modelClientOptions = modelClientOptions || {
       apiKey: process.env.GOOGLE_GENERATIVE_AI_API_KEY || "",
     };