feat(agui): propagate userId from forwardedProps to RuntimeContext

[ningmeng0503]

When an AG-UI client sends a userId inside RunAgentInput.forwardedProps, AguiAgentAdapter now copies it into the built RuntimeContext so that downstream agents, tools, and hooks can access
it via RuntimeContext.getUserId().

Changes

• Added FORWARDED_PROP_USER_ID_KEY = "userId" constant in AguiAgentAdapter.
• Updated AguiAgentAdapter.buildRuntimeContext(RunAgentInput) to read userId from input.getForwardedProps() and set it on RuntimeContext.Builder.userId(...).
• Empty or blank userId values are ignored.
• Existing behavior of storing the full forwardedProps map under agui.forwardedProps is preserved.
• Added unit tests in AguiAgentAdapterTest covering:
  • userId set from forwarded props
  • non-String userId converted to String
  • missing userId keeps RuntimeContext.getUserId() null
  • blank userId is ignored

Example

An AG-UI client can now forward the user identity like this:

{
  "threadId": "thread-1",
  "runId": "run-1",
  "forwardedProps": {
    "userId": "user-123"
  }
}

Inside the agent run, RuntimeContext.getUserId() will return "user-123".

Checklist

- [x] Change is localized to AguiAgentAdapter.
- [x] Existing RuntimeContext behavior is preserved.
- [x] Unit tests added and passing.
- [x] Spotless formatting applied.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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