fix(eval): preserve custom eval metadata

[he-yufeng]

Summary

• allow eval models to carry custom metadata fields instead of rejecting them as extras
• cover both top-level EvalCase metadata and nested SessionInput metadata in serialization

Fixes #5906

To verify

• PYTHONUTF8=1 uv run --with pytest pytest tests/unittests/evaluation/test_eval_case.py -q
• PYTHONUTF8=1 uv run ruff check src/google/adk/evaluation/common.py tests/unittests/evaluation/test_eval_case.py
• PYTHONUTF8=1 uv run pyright src/google/adk/evaluation/common.py
• PYTHONUTF8=1 uv run python -m py_compile src/google/adk/evaluation/common.py tests/unittests/evaluation/test_eval_case.py
• git diff --check