fix(sessions): persist event custom metadata

[he-yufeng]

Summary

• add custom_metadata as a first-class Event field
• let DatabaseSessionService v1 persist it through the existing event_data dump/restore path
• assert session round-trips preserve custom metadata across session backends

Fixes #6055

To verify

• PYTHONPATH=src python -m pytest tests\unittests\sessions\test_v0_storage_event.py tests\unittests\sessions\test_session_service.py::test_append_event_complete -q
• PYTHONPATH=src python -m pytest tests\unittests\test_runners.py::test_run_config_custom_metadata_propagates_to_events -q
• python -m py_compile src\google\adk\events\event.py tests\unittests\sessions\test_session_service.py
• python -m ruff check src\google\adk\events\event.py tests\unittests\sessions\test_session_service.py
• git diff --check