## Why
Enabling background rollout migration disabled the TUI's cached legacy resume shortcut, even when the rollout maintenance lock could prevent migration during resume.
## What changed
Allow embedded sessions to use the shortcut regardless of migration feature settings when they acquire the maintenance lock and revalidate that the thread still uses legacy history. Hold the lock through `thread/resume` so the response can safely include turns. Remove the session's cached startup migration setting.
## Testing
Update the regression test to cover all four combinations of startup and workspace migration settings, checking lock acquisition before metadata revalidation, a two-request resume, and nonempty history.
GitOrigin-RevId: cf525a6edb813050469dff39b83e5e61214485a1
ac192cd793