## Why
A newly started thread may not have materialized its rollout before the TUI
receives `ThreadStarted`. Trying to infer session state from that path can wait
through rollout reader retries even when a lifecycle response already provided
the authoritative session.
## What changed
- Preserve session state already stored for a known thread instead of inferring
it again from `ThreadStarted`.
- Continue updating agent-picker metadata from the notification.
- Restrict fallback session inference to newly observed `ThreadStarted`
notifications that do not already have session state.
## Testing
Added a startup test that verifies a known thread routes `ThreadStarted`
immediately, retains its configured session, buffers the notification, and
updates agent metadata when the rollout does not exist yet.
GitOrigin-RevId: 7ed98fb46df17566c4a61ac677f60fc8d94f7321
Charlie Marsh
bf2aee99c5