## Why
Changes to MCP inputs can require rebuilding the runtime before the next model
step. Start that work eagerly so the step is less likely to wait for it.
## What changed
- Add a per-session worker that prewarms dirty MCP state after configuration,
authentication, or explicit refresh changes.
- Coalesce queued refreshes and always rebuild from the latest session state,
while keeping exact model-step refreshes as the correctness path.
- Stop and join the worker during session shutdown.
## Testing
- Verify rapid refreshes skip superseded configuration and publish the latest
state.
- Verify MCP-relevant policy changes mark the runtime for refresh.
GitOrigin-RevId: 045ed406caff97dcbb483bc183fd16eae0c7d7ad
ef2d3edb95