[Question] Recommended way to pass server-side userId when using AG-UI starter with HarnessAgent?

## Context

We are building a multi-user platform with:

- **Backend**: Spring Boot + Spring Security (session/cookie auth)
- **Agent**: `HarnessAgent` (singleton, stateless engine with `AgentStateStore`)
- **Frontend**: CopilotKit → AG-UI protocol
- **Integration**: `agentscope-agui-spring-boot-starter` (`AguiRestController` / `AguiAgentAdapter`)

Per the [Context & AgentState docs](https://java.agentscope.io/v2/en/docs/building-blocks/context.html), `(userId, sessionId)` should be passed via `RuntimeContext` on each call. The [Quickstart](https://java.agentscope.io/v2/en/docs/quickstart.html) also shows `userId` / `sessionId` being set in the HTTP handler.

However, the AG-UI integration path seems different from the `HarnessGateway` / builder example (`ChatController` → `MsgContext.userId` → `RuntimeContext`).

## What we observed

1. **RC4 (`AguiAgentAdapter`)**: calls `agent.stream(msgs, options)` without `RuntimeContext`, so `threadId` is not mapped to `sessionId`.
2. **main (after #1895)**: `buildRuntimeContext()` sets `sessionId = threadId` and puts `RunAgentInput` into context — but still **no `userId`**.
3. Without `userId`, `ReActAgent` falls back to `__anon__/{sessionId}` slot keys, which breaks multi-user isolation when multiple logged-in users share the same AG-UI `threadId` namespace.

We understand `userId` should come from the **integration layer** (server auth), not from the frontend `forwardedProps` (untrusted).

## Questions

1. Is there an **official recommended pattern** for passing **server-side authenticated `userId`** into `RuntimeContext` when using the **AG-UI Spring Boot starter** with **HarnessAgent**?
2. Should we wait for / contribute to the hook mechanism proposed in #901 (`AguiRequestHook` + `forwardedProps`), or is there a better extension point (e.g. custom `AguiRequestProcessor`, `RuntimeContextCustomizer`)?
3. For production multi-tenant chat, do you recommend staying on AG-UI + starter, or switching to the **builder-style `HarnessGateway`** path (custom SSE, not AG-UI)?

## Related issues / PRs

- #751 — AG-UI sessionId not exposed during execution (persistence)
- #901 — Custom hooks via `forwardedProps` (open)
- #1895 / #1780 — RuntimeContext propagation for AG-UI (`sessionId` ← `threadId`)

## Our current workaround

Wrapping the registered `Agent` to inject `SecurityContext` → `RuntimeContext.userId` before delegating to `HarnessAgent.stream(..., ctx)`. This works but feels like integration-layer logic placed at the wrong abstraction level.

Any guidance or planned API for this scenario would be appreciated. Happy to contribute a PR if there is a preferred design.

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**Environment**: `agentscope-java` 2.0.0-RC4, Spring Boot 3.4, Java 17