fix: include grounding metadata in rubric judge prompt

[he-yufeng]

Summary

This updates the rubric-based final response quality evaluator so model-supplied grounding metadata is available to the LLM-as-judge prompt.

The issue is easiest to hit with model-internal tools such as google_search: the evaluator currently tells the judge to trust only function tool_response values, but those raw search results may not appear as normal function tool responses. ADK events can still carry grounding metadata, so this patch preserves that metadata in eval invocation events and serializes it into the judge prompt as trusted evidence.

Final answer text is still not treated as evidence.

Fixes #5831.

To verify

• python -m py_compile src/google/adk/evaluation/eval_case.py src/google/adk/evaluation/evaluation_generator.py src/google/adk/evaluation/llm_as_judge_utils.py src/google/adk/evaluation/rubric_based_final_response_quality_v1.py tests/unittests/evaluation/test_evaluation_generator.py tests/unittests/evaluation/test_llm_as_judge_utils.py tests/unittests/evaluation/test_rubric_based_final_response_quality_v1.py
• .venv\Scripts\python.exe -m pyink --check src\google\adk\evaluation\eval_case.py src\google\adk\evaluation\evaluation_generator.py src\google\adk\evaluation\llm_as_judge_utils.py src\google\adk\evaluation\rubric_based_final_response_quality_v1.py tests\unittests\evaluation\test_evaluation_generator.py tests\unittests\evaluation\test_llm_as_judge_utils.py tests\unittests\evaluation\test_rubric_based_final_response_quality_v1.py
• .venv\Scripts\python.exe -m isort --check-only src\google\adk\evaluation\eval_case.py src\google\adk\evaluation\evaluation_generator.py src\google\adk\evaluation\llm_as_judge_utils.py src\google\adk\evaluation\rubric_based_final_response_quality_v1.py tests\unittests\evaluation\test_evaluation_generator.py tests\unittests\evaluation\test_llm_as_judge_utils.py tests\unittests\evaluation\test_rubric_based_final_response_quality_v1.py
• .venv\Scripts\python.exe -m pytest tests\unittests\evaluation\test_eval_case.py tests\unittests\evaluation\test_llm_as_judge_utils.py tests\unittests\evaluation\test_rubric_based_final_response_quality_v1.py tests\unittests\evaluation\test_evaluation_generator.py -q
• git diff --check

I also ran targeted pylint on the touched files. It still reports existing module-wide style warnings in these evaluation tests/modules, but no unused-import or grounding-metadata-specific issue remains.