fix(eval): preserve custom eval metadata

[he-yufeng]

Summary

• allow evaluation models to preserve caller-provided metadata fields
• add a regression test covering extra fields on SessionInput and EvalCase

Fixes #5906

To verify

• PYTHONPATH=src python -m pytest tests/unittests/evaluation/test_eval_case.py -q
• python -m pyink --check src/google/adk/evaluation/common.py tests/unittests/evaluation/test_eval_case.py
• python -m ruff check src/google/adk/evaluation/common.py tests/unittests/evaluation/test_eval_case.py
• git diff --check

[he-yufeng]

Rebased this branch onto the latest main.

Validation on the rebased branch:

• PYTHONPATH=src .venv\Scripts\python.exe -m pytest tests\unittests\evaluation\test_eval_case.py -q
• PYTHONPATH=src .venv\Scripts\python.exe -m py_compile src\google\adk\evaluation\common.py tests\unittests\evaluation\test_eval_case.py
• git diff --check upstream/main...HEAD

Note: using the repo-local .venv is necessary on my Windows machine; the global conda environment has an older/incompatible google-genai install.