fix: extract model string for LlmRequest in simulator and per-turn evaluator

LlmRequest.model expects str but self._config.model and
self._llm_options.judge_model can now be BaseLlm instances.
Extract .model string before passing to LlmRequest.

Author: yuvrajangadsingh

src/google/adk/evaluation/simulation/llm_backed_user_simulator.py

@@ -180,8 +180,10 @@ async def _get_llm_response(
         user_persona=self._user_persona,
     )
 
+    config_model = self._config.model
+    model_str = config_model if isinstance(config_model, str) else config_model.model
     llm_request = LlmRequest(
-        model=self._config.model,
+        model=model_str,
         config=self._config.model_configuration,
         contents=[
             genai_types.Content(

src/google/adk/evaluation/simulation/per_turn_user_simulator_quality_v1.py

@@ -327,8 +327,10 @@ async def _evaluate_intermediate_turn(
         previous_invocations=invocation_history,
     )
 
+    judge_model = self._llm_options.judge_model
+    model_str = judge_model if isinstance(judge_model, str) else judge_model.model
     llm_request = LlmRequest(
-        model=self._llm_options.judge_model,
+        model=model_str,
         contents=[
             genai_types.Content(
                 parts=[genai_types.Part(text=auto_rater_prompt)],