Start rollout migration after runtime feature enablement (#40668)

## Why

App-server clients can enable `background_paginated_rollout_migration` after the server has started, so the startup-only migration path does not handle that transition.

## What changed

- Allow runtime enablement of `background_paginated_rollout_migration`.
- Start the local rollout migration when the feature transitions from disabled to enabled at runtime.

## Testing

Update the app-server integration test to enable migration through the runtime feature API, wait for paginated history, and verify that cold resume preserves model context.

GitOrigin-RevId: d695624a03b4f056be94fcaac2075f91896f19e5