fix: address gemini-code-assist review feedback

- Update LlmBackedUserSimulatorConfig.model type to Union[str, BaseLlm]
- Use isinstance check for model string extraction in LlmRequest
  instead of accessing .model on potentially mocked objects

Author: yuvrajangadsingh

src/google/adk/evaluation/hallucinations_v1.py

@@ -295,7 +295,7 @@ def __init__(self, eval_metric: EvalMetric):
     self.segmenter_prompt = _HALLUCINATIONS_V1_SEGMENTER_PROMPT
     self.sentence_validator_prompt = _HALLUCINATIONS_V1_VALIDATOR_PROMPT
     judge_model = self._judge_model_options.judge_model
-    self._model = judge_model.model if isinstance(judge_model, BaseLlm) else judge_model
+    self._model = judge_model if isinstance(judge_model, str) else judge_model.model
     self._model_config = (
         self._judge_model_options.judge_model_config
         or genai_types.GenerateContentConfig()

src/google/adk/evaluation/llm_as_judge.py

@@ -137,7 +137,7 @@ async def evaluate_invocations(
     for actual, expected in zip(actual_invocations, expected_invocations):
       auto_rater_prompt = self.format_auto_rater_prompt(actual, expected)
       judge_model = self._judge_model_options.judge_model
-      model_str = judge_model.model if isinstance(judge_model, BaseLlm) else judge_model
+      model_str = judge_model if isinstance(judge_model, str) else judge_model.model
       llm_request = LlmRequest(
           model=model_str,
           contents=[

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

@@ -17,6 +17,7 @@
 import logging
 from typing import ClassVar
 from typing import Optional
+from typing import Union
 
 from google.genai import types as genai_types
 from pydantic import Field
@@ -48,9 +49,12 @@
 class LlmBackedUserSimulatorConfig(BaseUserSimulatorConfig):
   """Contains configurations required by an LLM backed user simulator."""
 
-  model: str = Field(
+  model: Union[str, BaseLlm] = Field(
       default="gemini-2.5-flash",
-      description="The model to use for user simulation.",
+      description=(
+          "The model to use for user simulation. It can be a model name"
+          " string or a BaseLlm instance for custom/self-hosted models."
+      ),
   )
 
   model_configuration: genai_types.GenerateContentConfig = Field(