fix: preserve cache_metadata in VertexAiSessionService event round-trip#4714

weiguangli-io wants to merge 5 commits intogoogle:maingoogle/adk-python:mainfrom

weiguangli-io:codex/adk-python-4698-vertexai-cache-metadataweiguangli-io/adk-python:codex/adk-python-4698-vertexai-cache-metadataCopy head branch name to clipboard