Using VertexAiSessionService with Cloud Run (or GKE) deployed agent

The [ADK Cloud Run deployment docs](https://google.github.io/adk-docs/deploy/cloud-run/#options) lists `--agent_engine_id` as an option for `adk deploy cloud_run`, implying I can deploy an agent to Cloud Run while using Vertex AI Agent Engine Sessions as a managed session store.

However, obtaining an agent_engine_id currently requires creating an "empty" Agent Engine instance (via `agent_engines.create()`) or deploying an agent to Agent Engine runtime, which is not explained on Cloud Run page and feels confusing and unintuitive.

**Request:** Please document a preferred, production-grade workflow for Cloud Run (or any other runtime) + VertexAiSessionService setup.